Caramelized Onion & Gruyère Tart
The most patient onions you'll ever cook, locked into a flaky tart.
Don't rush the onions. There's no shortcut to deeply caramelized — only time, patience, and a wide pan. Sixty minutes feels long until you taste them, and then you'll wish you'd started with four pounds instead of three.
Good-quality all-butter puff pastry from the freezer aisle is more than fine. (I almost always use it.) Look for one with "butter" as the first fat on the ingredient list.
Method
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Caramelize the onions
Melt the butter with the olive oil in a wide heavy skillet over medium heat. Add the onions and salt — the pan will look impossibly full, but they collapse. Cook, stirring every few minutes, for 50–60 minutes, lowering the heat if anything starts to scorch. They'll go from white to translucent to golden to a deep, glossy mahogany. Stir in the thyme leaves and vinegar in the last 2 minutes, then take off the heat. Cool to barely warm.
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Shape the tart
Heat the oven to 400°F (200°C). Unfold the puff pastry onto a parchment-lined sheet pan. Score a ½-inch border around the edges with a knife — don't cut through. Prick the interior all over with a fork. Brush the border with egg wash.
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Top and bake
Spread the cooled onions evenly inside the border. Scatter with the Gruyère, finish with several twists of black pepper, and bake for 22–28 minutes, until the border has puffed and turned deep gold and the cheese is bubbling.
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Rest and slice
Let the tart rest 5 minutes on the pan — the puff sets up and stops being floppy. Finish with flaky salt, slice into squares, and serve warm.
Cook's notes
- A handful of arugula tossed with lemon and olive oil on top of each slice turns this into the kind of meal you wish you had at a wine bar.
From the comments
2 notes from people who cooked this.
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Margaux B.
I caramelized the onions while watching a movie. The activity was completely manageable. The tart was completely gone in twenty minutes.
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Theo P.
Used a mix of Gruyère and a sharper Comté and added some sliced fig on top before baking. Approved.
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