This French onion tart is an upside-down appetizer featuring caramelized red onions and nutty Gruyère cheese baked under flaky puff pastry, then inverted for an elegant presentation. The tart combines Paris bistro flavors in a simple, stunning dish.
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On a parchment-lined baking sheet, drizzle olive oil and hot honey.
Sprinkle with flaky sea salt, freshly ground black pepper, and fresh thyme sprigs.
Arrange the sliced red onions in a single layer over the seasoned oil.
Layer the slices of Gruyère cheese on top of the onions.
Drape the thawed puff pastry sheet over the cheese and onions.
Crimp the edges of the pastry with a fork to seal.
Brush the top of the pastry with the beaten egg.
Bake until the pastry is golden brown and crisp.
Carefully invert the tart onto a cutting board and peel off the parchment paper.
Drizzle with balsamic glaze and garnish with more flaky sea salt and fresh thyme leaves.
Slice and serve immediately.
Store leftover tart covered in the refrigerator for up to 2 days. Reheat in a 350°F oven for 8–10 minutes until warm, or serve at room temperature.
Yes. Yellow onions will work and caramelize similarly, though they lack the subtle sweetness and color of red onions. White onions are less recommended as they can become too soft.
No. The onions cook and caramelize during baking under the pastry. Simply slice and layer them raw as the recipe directs.
Bake until the visible edges and top of the pastry turn a deep golden brown and appear crisp. This typically takes 20–25 minutes, but check visually as oven temperatures vary.
Yes. Assemble the tart on parchment up to 4 hours before baking, cover, and refrigerate. Bake when ready. Serve warm or at room temperature the same day for best texture.
Let the tart cool for 1–2 minutes after baking so the cheese sets slightly, then invert quickly and confidently onto a cutting board. If pieces stick, gently coax them free with a spatula.
