Easy Upside Down French Onion Tart

Name: Easy Upside Down French Onion Tart
Prep time: 20 min
Cook time: 30 min
Total time: 50 min
Yield: 6
Cuisine: French
Ingredients: 2 small red onions, sliced into thin rings; 2 tablespoons olive oil; 4 tablespoons honey or hot honey; 1/2 teaspoon flaky sea salt, plus extra for finishing; 1/2 teaspoon black pepper; 6 fresh thyme sprigs; 3 ounces Gruyère cheese, thinly sliced; 1 sheet frozen puff pastry, thawed; 1 large egg, beaten with 1 teaspoon water; Balsamic glaze for drizzling (optional)

A crispy, cheesy tart made with sweet caramelized onions and buttery puff pastry.

By Enzo81 Kitchen· French· October 19, 2026

enzo81.xyz

Prep

20 minutes

Cook

30 minutes

Total

50 minutes

Serves

I love how simple ingredients can turn into something truly elegant without much fuss. This onion tart is a favorite of mine because it uses a clever upside down trick to get that perfect caramelization without spending an hour at the stove.

The combination of sweet honey, salty Gruyère cheese, and fresh thyme is always a hit at gatherings. To make things even easier, I recommend using a mandoline to get those onions perfectly thin and even before you start layering.

Ingredients

2 small red onions, sliced into thin rings
2 tablespoons olive oil
4 tablespoons honey or hot honey
1/2 teaspoon flaky sea salt, plus extra for finishing
1/2 teaspoon black pepper
6 fresh thyme sprigs
3 ounces Gruyère cheese, thinly sliced
1 sheet frozen puff pastry, thawed
1 large egg, beaten with 1 teaspoon water
Balsamic glaze for drizzling (optional)

10Instructions

1
Preheat your oven to 400 degrees and line a large baking sheet with parchment paper.
2
Slice your onions into thin rounds about a quarter inch thick, keeping the rings together as much as possible.
3
Make six small patches of olive oil on your parchment paper, arranged in two rows.
4
Drizzle the honey over the oil patches and sprinkle them with salt, pepper, and a sprig of thyme.
5
Place your onion rounds over the honey mixture, filling in any gaps with smaller pieces so the parchment is mostly covered.
6
Lay the Gruyère slices over the onions, keeping the cheese away from the very edges of each patch.
7
Unfold your puff pastry and cut it into six rectangles that match the size of your onion patches.
8
Place each pastry rectangle over a cheese and onion patch, pressing the edges down gently.
9
Brush the tops of the pastry with your egg wash and bake for 25 to 30 minutes until deep golden brown.
10
Let the tart cool for a few minutes, then carefully flip the pieces over so the onions are on top and finish with a drizzle of balsamic glaze.

Tips

If you cannot find Gruyère, Swiss or Emmental cheese works just as well for that classic nutty flavor. Make sure your puff pastry is still cold when it goes into the oven to get the best rise. Store any leftovers in an airtight container and reheat them in the oven to keep the pastry crisp.

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