This onion tart is a tender and delicious puff pastry topped with caramelized onions makes the perfect appetizer or side dish! It is insanely delicious and worth being served at any party or get-together!

I absolutely love a good appetizer because I love a good party! And the food is what makes it or breaks it! Try out this fantastic 5 minute dip, these delicious egg rolls, or these chicken wings!

The top view of a cooked onion tart sliced into squares.

What is an Onion Tart?

An onion tart is a fantastic finger food appetizer that can be served for any occasion and any guest! You start out with a large puff pastry, and you top it with some onions that have been cooked and caramelized in sugar. Once you add the caramelized onion to the puff pastry, you add some cheese and thyme, and then you bake them together! Slice it up and serve it to your guests on a platter!

Describing this as an appetizer is nothing compared to eating it! The puff pastry cooks to the perfect golden brown, which gives a slight crunch when you bite it. When you combine that with the deliciously flavored and caramelized textured onions, well, it’s game over. It melts in your mouth, and you literally won’t be able to stop yourself from coming back for seconds! Don’t miss out! Cook this for your next dinner party, and your guests will go crazy over it!

What Ingredients are in an Onion Tart Recipe?

The ingredients in this recipe are simple and easy! In fact, you may already have most of them ready to go in your kitchen! Grab anything that you don’t already have at the store and get cooking! Your tastebuds are waiting! Check out the recipe ard at the bottom of the post for exact measurements.

  • Puff Pastry: Tkae the time to thaw your puff pastry in the fridge, overnight.
  • Sweet Yellow Onions: Slices these up into thin , even pieces so that that they cook together at the same time.
  • Olive Oil: I used regular olive oil to cook the onions in.
  • Brown Sugar: This is not only delicious but necessary to get that caramelized glaze on the onions.
  • Kosher Salt: This will enhance all of the flavors in this recipe.
  • Gruyere Cheese: If you don’t have this cheese then you can try gouda, feta or mozzarella instead!
  • Fresh Thyme: I like to use fresh thyme because it has a more potent and delicious taste.
  • Heavy Cream: This is what you will use to get that golden brown edge on the puff pastry!

How to Make and Onion Tart

This delicious recipe takes less than an hour from start to finish. And that includes all of the cooking time. Except this tastes better! It’s a great go-to for any appetizer options and feels fancy, like something that you would order at a restaurant…

  1. Cook onions: Heat oil in a large deep pan on medium-high heat. Stir in the onions and cook for about 10 minutes or until softened and wilted. 
  2. Caramelize: Add the salt and brown sugar and cook uncovered for an additional 20 minutes until the onions are caramelized. Set aside and allow to cool while you prepare the tart. Make sure your caramelized onions are cooled enough before you put them on the cold tart.
  3. Preheat: Preheat the oven to 400° Fahrenheit.
  4. Prep pastry puff: Carefully unroll each pastry tart. Place them slightly overlapping on a baking sheet lined with parchment paper. Lightly press the two tarts creating one large rectangular-sized tart. Fold up the sides around the whole tart to create a pretty finished edge. Prick with a fork every inch or so to help prevent bubbling while baking.
  5. Add cheese: Sprinkle the cheese evenly on the tart. 
  6. Add onion mixture: Evenly lay the onions on top of the pastry.
  7. Brush with cream: Using a pastry brush, brush the folded edges of the pastry with heavy cream to create that golden brown crust. Sprinkle on the thyme leaves.
  8. Bake: Bake for 18-20 minutes or until cheese has melted and pastry is crisp. Cool for 5 minutes before cutting the caramelized onion tart into squares or triangles. Garnish with additional thyme leaves if desired.
6 pictures showing how to cook caramelized onions, how to add them to a puff pastry and get it ready to go into the oven.

Helpful Tips

I found a lot of success using the following tips when making the onion tart. The recipe can seem intimidating, but if you follow these tips, you will find success in making the tart!

  • Thaw: Don’t forget to plan ahead to thaw the puff pastry dough in the refrigerator the night before.
  • Keep it cool: Keep the puff pastry refrigerated until ready to use. The chilled pastry creates a flakey result.
  • Prick it: Make sure to prick the pastry with a fork to prevent the pastry from bubbling.
  • Cooking onions: Cooking the onions low and slow is best for bringing out the sweetness. Be patient! Allow the onions to cool for about 10 minutes before assembling. This way the pastry stays chilled enough before baking.
  • Cheese options: Gorgonzola, brie, blue, gouda, feta, mozzarella, swiss chees or really any cheese you think would pair well with the flavors.
  • Use parchment paper: Parchment paper is your best friend when working with puff pastry. It also makes cleanup easy!
  • Tips for serving: The tart can be sliced into squares, strips, or triangles. It serves beautifully any way!
  • Garnish ideas: I love to garnish with fresh herbs before serving. A drizzle of balsamic glaze is also a crowd pleaser!

The top view of a fully cooked onion tart, just out of the oven.

How to Store and Reheat Leftovers

Once your onion tart has cooled, then you can place it in an airtight container and put it in your fridge. It will last 4-5 days. I recommend reheating it in the oven at 375 degrees Fahrenheit for about 15-20 minutes or until it’s heated through.

A slice of onion tart on a flate with a silver fork.

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