Caramelized Onion Puff Pastry Bites with sautéed crimini mushrooms, balsamic caramelized onions, and applewood smoked gruyere cheese. The perfect little appetizers! They’re made with puff pastry and take no time at all to whip up. They are the perfect appetizers to serve your guests this holiday season!

caramelized onion puff pastry squares on serving plate


Buttery, flaky puff pastry topped with balsamic caramelized onions, sauteéd mushrooms, a hint of garlic, and a smoked gruyere cheese bomb mixed right in. 

These caramelized onion bites are a game-changer for me. They’re perfect to serve your Thanksgiving, Christmas, and New Years guests before a big dinner. Also, they totally look fancy schmancy, but nobody really needs to know that the caramelized onion and sauteéd mushroom filling can be made a couple of days ahead of time. 

I’ve had readers tell me that these easy caramelized onion puff pastry bites flew out of the oven and right onto the guests’ plates! They are always a hit when we make them for parties, and with a sprinkle of parsley or some fresh thyme, the scent of these caramelized onion bites is UNREAL.

How to make caramelized onion bites:

platter or onion puff pastry bites

Ingredients for caramelized onion tarts (or tartlets)

  • Butter and oil: We’ll use these ingredients to saute the mushrooms and the onions for the recipe. I like to use a high-heat oil such as avocado, sunflower, or vegetable to cook the onions and mushrooms.
  • Mushrooms: I prefer to use small crimini or baby bella mushrooms, but button mushrooms, shiitakes, or a medley of mushrooms would also work for this recipe.
  • Yellow onions: slice the onions thinly so that the caramelizes quicker in the pan. We’ll cook them down in a large skillet until they are tender and delicious.
  • Dried or fresh thyme: We’ll use some type to flavor the onions.
  • Seasonings: I keep things pretty simple. You’ll want some sugar to help the caramelization process of the onions; you’ll also need kosher salt, black pepper, and some garlic powder.
  • Balsamic Vinegar: The syrupy vinegar cooks down with the onions and creates a balsamic glaze in the pan. This not only gives the onions a more delicious color, it also helps boost the sweetness which pairs beautifully with the nutty gruyere cheese.
  • Gruyere Cheese: I like to use applewood smoked gruyere for this recipe as the smoky flavor pairs well with the sweetness of the jammy onions and the natural umami or mushrooms. You can also use regular gruyere or other economical options like white cheddar.
  • Puff Pastry: For this recipe, you’ll need a box of frozen puff pastry. To make the process smoother, I like to thaw the puff pastry in the refrigerator overnight according to the package directions. I’ve never been able to get puff pastry to defrost quickly, so definitely plan ahead for this recipe.
  • Large egg: This is for the egg wash! It just gives the puff pastry a beautiful sheen when it comes out of the oven.
mushrooms, onions, butter, balsamic, salt, and thyme on baking sheet
puff pastry squares before baking

How to make caramelized onion puff pastry bites:

  1.  Start with the mushrooms. We’re going to start by sautéing the mushrooms in a few tablespoons of oil for 4-5 minutes. Once they start to brown, remove them to small bowl.
  2. Caramelize the onions. Add the remaining butter to the pan. Once it melts, add the onions. Let them hang out in the pan for 5 minutes. Remember to use a cast iron or stainless steel skillet. You also want to avoid using a nonstick skillet as the onions won’t caramelize. After the 5 minutes, season ith sugar and thyme. Now crank the heat down to about medium-low, and cook for a total of 25 minutes. Keep checking on them every 5-10 minutes, and give them a stir to keep them from browning too quickly. Drizzle the balsamic to help deglaze the pan, and when it’s all been soaked up, you can turn off the stove. Once the onions have cooled slightly, add them to the mushrooms and season with a little garlic powder, salt, and pepper. 
  3. Store or use right away. Let your filling come to room temperature, you can store it in an air-tight container for up to 48 hours in the refrigerator. If you decide to make the filling ahead of time, zap it in the microwave to bring it back to room temp before continuing with the recipe instructions. Mix in the gruyere cheese and combine well.
  4. Make the bites. Cut the puff pastry sheet into small squares or rectangles using a pizza cutter or a sharp knife. I cut mine into 2-inch squares. Lay a piece of parchment paper on a large baking sheet. Place the squares on the prepared sheet pan. Brush them with a beaten egg using a pastry brush. Drop about a tablespoon of caramelized onion mixture on each puff pastry square.
  5. Bake: bake the caramelized onion puff pastry bites in the preheated oven until they are crisp and golden brown. Let them cool slightly before serving. Top with fresh parsley or thyme for a little color!
prepared appetizers on baking dish
puff pastry topped with onions and mushrooms

Look at each of those puff pastry layers and how perfectly they separate from each other. It’s like taking a bite out of 10 potato chips at the same time, only less crunchy, and more buttery. Okay, so not like a potato chip at all, really.

Caramelized onion puff pastry bites are going to make your holidays more amazing. The garlic, the mushrooms, the caramelized onions, and the smoky gruyere. 

If you like this caramelized onion bites recipe, you might also like:

puff pastry topped with caramelized onions and parsley on dish
Yield: makes 30-34 bites

Caramelized Onion Puff Pastry Bites with Gruyere and Mushrooms

Prep Time 15 minutes
Cook Time 45 minutes
Total Time 1 hour

Caramelized Onion Puff Pastry bites with sautéed crimini mushrooms, balsamic caramelized onions, and applewood smoked gruyere cheese. The perfect little appetizers! They’re made with puff pastry and take no time at all to whip up! These are the perfect appetizers to serve your guests this holiday season.

Caramelized Onion Puff Pastry Bites with Gruyere and Mushrooms


  • 4 tablespoons salted butter, divided
  • 2 tablespoons vegetable oil, divided
  • 5-6 ounces crimini or button mushrooms, sliced
  • 2 medium yellow onions, thinly sliced
  • 1/4 teaspoon dried thyme
  • 1 tablespoon sugar
  • 1 1/2 tablespoon balsamic vinegar
  • 1/4 teaspoon garlic powder
  • salt and pepper, to taste
  • 4 ounces grated gruyere cheese (I used applewood smoked gruyere)
  • 1 package (1 pound) frozen puff pastry, thawed
  • 1 egg, lightly beaten


  1. MUSHROOMS: In a large cast iron skillet over medium heat, heat 1 tablespoon of oil along with 1 tablespoon of butter. When the butter melts, add the mushrooms and sauté for about 5 minutes until golden and tender. Remove and set aside in a medium bowl.
  2. CARAMELIZE: Heat the remaining 3 tablespoons of butter along with 1 tablespoon of oil and sauté the onions for 5 minutes. Add the dried thyme and sugar and continue to cook for 25 minutes on medium-low heat. Make sure to stir the mixture every 5-10 minutes. If the onions are caramelizing too fast, turn down the heat a little more.
  3. DEGLAZE: Add the balsamic vinegar to deglaze the pan, allow the onions to soak it all in. Turn off heat. Add the onions to the mushroom mixture. Season with the garlic powder and salt and pepper to taste, stir to combine.
  4. MAKE AHEAD: Allow to cool to room temperature. The mixture can be kept in an air-tight container in the fridge for up to 48 hours. Allow to come to room temperature before proceeding. Add the gruyere cheese and mix well.
  5. PREP FOR BAKING: Position two racks in the upper and lower thirds of the oven. Preheat the oven to 400º F. Line 2 baking sheets with parchment paper or silicone mats.
  6. FILL: Using a sharp knife or a pizza cutter, cut the puff pastry into 2-inch squares and place them on a lined baking sheet. Make sure the puff pastry is cold before proceeding. If not, allow it to rest in the refrigerator for 10-15 minutes before proceeding. Using a pastry brush, brush the beaten egg on the squares. Prick the puff pastry using a fork in the center. Place a tablespoon of caramelized onion mixture in the center.
  7. BAKE: for 20-25 minutes, rotating the pan halfway in between until crisp and golden brown. Let cool slightly before serving.


  • Make sure the puff pastry is cold when it goes into the oven. If at any point it becomes soft, allow it to rest in the refrigerator for 15-20 minutes or until it firms up again. Do not open the oven door during the first 10 minutes of baking time. Frequently opening the oven may cause the puff pastry to not 'puff up'.
  • Mozzarella or gouda cheese may be used to replace the gruyere. I really like the smoky flavor of applewood smoked gruyere with these bites.
  • If you've prepared the caramelized onion mixture ahead of time, you can zap it in the microwave for 30 seconds to bring it back to room temperature quickly.

Have you made this recipe?

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