This easy onion tart layers thinly sliced onions, Gruyère cheese, and thyme on puff pastry, then bakes them into an elegant appetizer perfect for the holidays, or any party you want to feel extra special!
Onion tart: humble ingredients, delicious results!
French cooking has the reputation of being fussy…and some of it is.
I’ve traveled all over France in the last 25 years, however, and I can tell you that much of it really is simple.
In fact, the French dishes I love most are the ones that transform a humble ingredient, like onions, into something special.
When we were in Alsace last year, I fell in love with tarte à l’oignon. It’s little more than pastry, onion, cheese, and cream, but WOW, the combined effect is spectacular.
French onion soup is another example. It doesn’t get more rustic than onions, cheese, and bread, yet the soup is so complex, delicious, and satisfying, it’s hard to want anything more.
(If you’d like to try French-style cooking at home, check out my French Onion Soup recipe. It takes some time, but it’s easy and sooooo worth it!)
How to Make a Faster Caramelized Onion Tart
The key to developing onions’ flavor is the caramelization.
Unfortunately, traditional caramelized onions are time-consuming. It can take 30 minutes to 1 hour, depending on your batch size.
To cheat the clock, I sliced the onions super thinly on a mandoline, layered them on a baking sheet with honey to mimic the caramelized sugar, then baked the puff pastry ON TOP of the onions.
The onions caramelize against the pan, then when the tart is done baking, you flip it over and slice.
It’s the same method I used for this Upside Down Apple Tart. It works like a dream!
Key Ingredients
You’ll find the full list of ingredients in the recipe card below, but here are some notes to keep in mind.
- Red onions. I picked red for color, but yellow is fine too. The most important thing about the onions is that you slice them thinly. A mandoline slicer is really and truly the move. It’s one of those gadgets that you don’t need every day, but when you do, nothing else can replace it.
- Gruyère cheese. This is the traditional cheese for French onion soup, but any nutty, melty, Alpine-style cheese is great. Emmental is an excellent, readily available swap. Munster is traditional in Alsace and works well too.
- Puff pastry. This is an ingredient where you really get what you pay for. I like Pepperidge Farm.
- Fresh thyme. Don’t substitute dried, or the flavor will be too strong.
- Flaky sea salt. Puff pastry on its own is bland, so the salt is needed. You can also use kosher salt, but I love the texture and appearance of the larger flakes.
More Excellent Appetizers with Puff Pastry
These are easy, but your friends will be very impressed!
Onion Tart
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Ingredients
- 2 small/medium red onions
- 2 tablespoons extra-virgin olive oil
- 4 tablespoons honey or hot honey
- ½ teaspoon flaky sea salt plus additional for serving
- ½ teaspoon ground black pepper
- 6 to 8 sprigs fresh thyme plus fresh leaves for serving
- 3 ounces Gruyère cheese thinly sliced
- 1 sheet frozen puff pastry thawed according to package directions
- 1 large egg beaten with 1 teaspoon water to create an egg wash
- Balsamic glaze optional for serving
Instructions
- Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
- Peel the onions, then slice them crosswise 1/4-inch thick through the equator (not the ends) so that you create rings. I HIGHLY recommend a mandoline for easy, even slicing. Try to keep the layers in each slice together so you have onion "discs."
- Create 6 drizzle patches of olive oil spaced evenly over the parchment in 2 rows of 3 (see video for suggested placement).
- Drizzle the honey over the oil, then sprinkle each patch with a pinch of flaky salt and black pepper. Lay a small thyme sprig across each one.
- Find your largest onion discs and lay one over each patch. Then, take the smaller discs and fill in the gaps. Almost no parchment should peek through; a little overlap is fine.
- Lay the Gruyère slices over the onions, spacing them evenly and leaving a border of at least 1 inch on all sides.
- Unfold the puff pastry on a lightly floured work surface. Roll it into a rectangle about 2 inches longer than the onions on each side (mine was 10 by 13). Gently drape the pastry over the onions. With the tines of a fork, press the edges of the pastry all the way around to create a border.
- Brush the pastry lightly with the egg wash. Dab up any pools that collect with a paper towel.
- Bake the onion tart for 25 to 30 minutes on the center rack, until the pastry is deeply golden and puffed.
- Let rest 5 to 10 minutes. Set a large cutting board on top of the pan. Working quickly and confidently, flip the board and tart over together so the tart is now sitting on the board. Lift off the pan and peel away the parchment. Drizzle with balsamic glaze, scatter with fresh thyme leaves and flaky salt, and slice into squares.
Notes
- Don’t stress over the tart being hot from the oven the moment you serve it. It tastes great at room temperature also.
- This tart tastes best the day it is made but can be stored in the refrigerator overnight. Rewarm in the oven, toaster oven, or air fryer at 375°F.
