This caramelized onion tart is made with flaky puff pastry (don't worry, we're using store-bought!), slow-cooked and perfectly caramelized onions, and gooey gruyère cheese. It's like having a bowl of French onion soup in a tart! All the warm, savory flavors in an easy-to-serve appetizer that's perfect for hosting.
The key to this recipe is properly caramelizing the onions. When you cook the onions low and slow with butter and thyme, they develop a deep golden color and rich, sweet flavor, just like when making French onion soup from scratch. This step takes patience, but it's what gives the tart its incredible depth of flavor.
Serve this onion tart as an appetizer, alongside a soup like my carrot ginger soup or creamy mushroom soup, or as a light lunch with a simple salad. It's also perfect for entertaining. Your guests will love these! Just be warned, they go quickly!
Looking for more puff pastry recipes? Try these easy mushroom puff pastry bites, as well!
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Allergens:
Wheat, Dairy (Butter and Cheese)
Ingredients
- Puff pastry - I use store-bought puff pastry for this recipe. Trader Joe's puff pastry is a favorite (It's a seasonal item, so I stock up and freeze it), but any brand will work great.
- Onion - You can use regular yellow onions, sweet onions like Vidalia, or a combination of both. And remember, take your time when caramelizing. Don't rush the process or you'll miss out on that deep, sweet flavor!
- Thyme - Use fresh thyme whenever possible. It adds a bright, herbal flavor that really complements the onions.
- Cheese - Gruyere is the classic cheese used in French onion soup, and for good reason! It grates easily, melts nicely, and is not overpowering. An alternative would be Emmental or Comte cheeses, but Gruyère is my first choice.
- Butter + oil - Using both gives the best results to sweat and ultimately caramelize the onions. The butter adds flavor and richness, while the oil, with its higher smoke point, prevents burning and helps with browning.
See recipe card for quantities.
How to make the caramelized onion filling
- Heat a heavy-bottomed pan over medium heat. (I love using a stainless steel pan for caramelizing onions. Check out my guide on how to cook with stainless steel if you're new to it!) Add oil, then butter, and let it melt. Add thinly sliced onions and stir until coated and softened (about 5 minutes). Sprinkle with salt and add fresh thyme.
- Let the onions caramelize by cooking them slowly over low to medium heat, stirring occasionally with a spatula. Add a bit more butter or oil if the pan gets too dry. (It should be coated but not swimming in butter/oil). This process takes about 30 to 45 minutes and requires patience. While you wait, curl up with a good book or sip your favorite cup of tea! Ultimately, the onions should be soft, fragrant, and deep golden brown. Don't rush it!
- Once the onions are perfectly caramelized, remove them from the heat and let them cool completely before assembling the tart.
Two ways to make this onion tart
Now that the caramelized onions are ready, make sure your puff pastry is defrosted, but still cool. You can assemble the tart in one of two ways: as a single large tart that is cut into slices before serving, or as bite-sized pieces.
Option 1: One Large Tart
- Roll out the entire puff pastry sheet and place it on a parchment-lined baking sheet.
- Using a fork, gently poke holes all over the puff pastry. This prevents it from puffing up too much while baking.
- Spread the cooled caramelized onions evenly over the puff pastry, leaving a small border around the edges (about an inch).
- Sprinkle the grated gruyere cheese evenly over the onions.
- Fold the edges inward over the filling to create a border crust.
- Bake on the middle rack for about 10-12 minutes, or until the pastry is golden brown. Once cooled, slice into squares or triangles for serving.
Option 2: Individual Puff Pastry Bites
- Roll out the puff pastry dough. Using a small knife, cut it into 12 equal squares. An easy way to do this is to cut three rows down and four rows across, creating a 4x3 grid.
- Add a spoonful of cooled caramelized onion filling to the center of each square, then top with grated gruere cheese.
- Fold two opposite corners toward the center to create a pocket, or fold one corner over to the opposite side, creating a closed triangle. Place on parchment-lined baking sheet.
- Bake on the middle rack for about 10 minutes, until the pastry is golden brown.
Recipe Tips
- Remember to defrost the puff pastry ahead of time so it's ready to use when needed.
- Keep the puff pastry cool so that it's easier to work with. If it becomes a bit warm, it might be hard to work with; let it chill in the fridge.
- Allow the onion mixture to cool down before adding it to the puff pastry.
- You can make the onion mixture a day ahead if needed and store it in the fridge.
What to serve with onion tart
This onion tart is incredibly versatile! Serve it as an appetizer at your next gathering. The individual bites are perfect for parties and always disappear quickly.
Pair it with a bowl of soup, such as my salmon soup or green borscht (spinach soup), for a cozy lunch, or serve it alongside a fresh salad, like my citrus salad with fennel or a grilled peaches, arugula, and halloumi salad. It also makes an elegant starter before main courses, such as whole-roasted branzino or barramundi fillet.
Leftovers and Storing
This puff pastry onion tart is best served right away while it's still warm. It can become a bit soggy as it sits. However, if you have leftovers, you can transfer them to an airtight container and store them in the refrigerator for 2 to 3 days.
When ready to serve, reheat in the oven just until warmed through.
FAQ
Emmental cheese, another type of Swiss cheese, or Comte could work as a Gruyère substitute. However, if possible, I highly recommend sticking with gruyere as it will replicate the traditional flavors of French onion soup.
Sautéed onions are cooked at a higher temperature. As a result, they become browned more quickly and take on a subtly sweet, almost charred flavor. On the other hand, carameilze dioniosn cook long and slow over fairly low heat, which draws out their natural sugars and creates a sweeter taste.
Entertaining
Looking for other recipes that are perfect for entertaining? Try these:
Pairing
These are my favorite dishes to serve with onion tart:
Caramelized Onion Tart with Gruyere
Ingredients
- 1 sheet puff pastry, Trader Joe's puff pastry works great here
- 2 large onions (yellow or sweet), thinly sliced
- 1 tablespoon olive oil
- 1 tablespoon butter
- 4 sprigs fresh thym
- ¼ teaspoon salt
- ½ cup gruyere cheese, grated
Instructions
Onion Filling
- Remove puff pastry from freezer and let thaw in the refrigerator overnight, or on the counter for 1 to 2 hours. Keep refrigerated until ready to use.
- Heat a heavy bottom pan over medium heat. Add oil, then butter, and let it melt.
- Add onions and stir until coated and softened, about 5 minutes. Sprinkle salt and add thyme.
- Cook onions over low to medium heat for about 30-45 minutes, stirring occasionally. Add more butter or oil if needed thruughout.
- Once the onions are soft, fragrant, and deep golden brown, remove from heat and cool completely.
Assemble the Tart
- Preheat oven to 400℉
- Roll out puff pastry on a parchment lined baking sheet.
- Use a fork to gently poke holes throughout the puff pastry.
- Spread cooled caramelized onions evenly over pastry, leaving a 1-inch border around edges.
- Sprinkle grated Gruyere cheese evenly over the onions. Fold the edges inward over the filling to create a border crust.
- Bake on the middle rack for 10-12 minutes, or until golden brown.
- Once cooled, slice into squares or triangles and serve.
Notes
- Keep puff pastry cold until ready to use
- Don't rush the caramelization process , patience is key!
- Onion filling can be made a day ahead and stored in the fridge.
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