This cheesy caramelized onion tart is just like having a bowl of french onion soup, but on a tart. The flavours of the caramelized onions, garlic and Gruyère cheese are so delicious together. The puff pastry is crunchy, buttery and the perfect vehicle for enjoying this tart.

If you're looking to enjoy a savory tart, let this caramelized onion tart be it. You can easily prep the more time consuming parts of this recipe ahead of time, in fact I'd recommend doing that anyways. And once you have all those components cooked and ready to go, this tart only takes 30 minutes to make.

Why You'll Love this Caramelized Onion Tart

• The taste is absolutely fantastic, smells amazing and is just like a comforting bowl of french onion soup.
• It can easily be prepped ahead of time.
• You can freeze leftovers easily, including any leftover caramelized onions.
• The crunchy, buttery tart is the perfect appetizer to share.
• Cheesy, oniony and garlicky, but without the harshness to it, because they're cooked down.

Ingredients

This french onion tart doesn't require a lot of ingredients. Here's everything you'll need.

• extra virgin olive oil
• butter to caramelize the onions
• yellow onions
• garlic
• puff pastry
• Gruyère cheese
• fresh thyme, removed from the stem

How to Make Caramelized Onions

In order to caramelize the onions correctly, you need some time. Some recipes say to cook the onions for only 20 minutes, which only takes the onions so far. Sure, they'll be softened, but the flavour won't be as strong, unless you really give the onions some time to develop that flavour.

Heat up your large skillet (I like using a large cast iron skillet or large dutch oven), over medium heat. Once it's hot, extra virgin olive oil and the butter. Then add in the onions. Stir often for about five minutes, and then reduce the heat to low. Add a pinch of sugar to help with the caramelization process if you wish, I personally don't do this.

This is what they will look like about halfway through.

Let it go for about 45 minutes to an hour (I like going the full hour). Only stir every now and then. If you stir too often, they won't get as browned as they can.

While the onions are cooking, you can roast the garlic head in the oven. Drizzle the garlic with a little bit of olive oil. Wrap in aluminum foil (or use parchment) and bake for 40 to 45 minutes, until softened. Let cool and then pop the cloves out of their paper and set aside.

How to Make This French Onion Tart

Line a large baking sheet with parchment paper.

Lightly flour a work surface area. Roll the puff pastry into a large rectangle (about 10 by 16-inches) and transfer to the baking sheet. Use a knife to run a 1-inch border around the edges, being careful not to pierce all the way through. Use a fork to poke holes throughout the center of the tart all over. This helps prevent bubbling while baking.

Scatter the cheese all over the tart.

Add the roasted garlic to the tart and then top with the onions and thyme. Brush the edges of the tart with milk or use an egg wash.

Bake for 20 minutes. Increase the temperature to 400ºF and bake for six to eight minutes, until the edges are browned, puffy and crispy.

Let cool slightly before slicing. Serve and enjoy!

If you like, drizzle a little balsamic on top after.

Tips to Make the Best Ever Caramelized Onion Tart

• Make ahead: as I suggest, make the onions and garlic ahead to save time, and to make things easier. The onions and garlic need to be cooled before going on the tart. I prefer to do this the night before and then store them in the fridge until I'm ready to assemble the onion tart.
• Be sure to defrost the puff pastry: take it out the night before to be sure it's defrosted.
• Don't overload the middle of the tart: the middle will get a little softer than the end or corner pieces. I suggest going a little lighter with the cheese and onion in the center so those center pieces holds up easier.
• Poke holes in the tart: creating the border with a knife and poking holes in the center of the puff pastry with a fork are helpful to ensure the puff pastry doesn't puff up too much in the center and allows the edges to be more distinct.
• Watch the tart carefully: during the last few minutes of baking, keep an eye on the tart to make sure it's getting browned, but not overcooked, or too browned. You can broil for a quick minute if you want at the end, but be careful and watch it to be sure it doesn't burn.
• Use a high quality puff pastry: personally, I find using a high quality puff pastry to be much tastier than ones made without real butter. If you have a bakery near you that sells puff pastry, even better! Personally, that's what I do.
• Use parchment: this makes handling the puff pastry and cleanup a lot easier.

Leftovers, Storage & Prep

• Store cooled leftover slices in a sealed container or ziploc bag in the fridge for a few days. Reheat leftover slices in the oven until hot.
• To freeze: let the slices cool completely and then store in a plastic freezer safe bag for up to three months.
• To reheat from frozen, bake in the oven at 350ºF for 15 minutes, until hot.
• Prep ahead: you can assemble the tart a few hours ahead of time, place in the fridge and then bake when you're ready.
• Leftover cooked onions: caramelized onions can easily be stored in the fridge in a sealed container for up to one week, or in the freezer in a freezer safe plastic bag for months.

If you have leftover caramelized onions, I highly suggest using them to make this french onion dip recipe. It's so good and a great appetizer to share!

Do I Need to Pre Bake the Puff Pastry?

Some people recommend pre-baking the puff pastry as it will help avoid a soggy bottom.

From personal experience, I didn't notice a big difference in doing so with this caramelized onion tart recipe. As suggested in the tips section, be sure to scatter the toppings more thinly in the center of the puff pastry to avoid sogginess.

If you want to be sure it's crisper and crunchier, pre-bake in the oven (without the toppings) for six to seven minutes. Then add the cheese and onions and bake for 12 to 15 minutes.

More Appetizer Recipes

Looking for more appetizers, check these ones out!

• Air fryer calamari
• Clue Cheese Stuffed Pears
• Prosciutto Wrapped Persimmons
• Air Fryer Brie Bites with Fig Jam
• Vegan Cashew Queso Dip
• Air fryer coconut shrimp with Thai sweet chili sauce
• Harissa & squash red lentil dip  (vegan)
• Air fryer Jalapeño Poppers
• Healthy french onion dip
• Butternut squash hummus with fried sage and dukkah
• Bang Bang Shrimp (air fryer)

Ready to make this? Let's get to it!

If you make this caramelized onion tart with cheese please share with me, by tagging me on Instagram or leave a comment and/or review below. I truly love seeing and hearing form you!

This post contains affiliate links, which means I may receive a small commission at no extra cost to you. I only share products and services I have personally used and love. Disclosure here.