Caramelized Onion Tart
This tart relies on the contrast between the crisp, flaky pastry and the dense, jammy sweetness of onions reduced over low heat. It is a lesson in how time changes the character of a simple ingredient.
Patience is your primary ingredient.
Do not rush the onions; they need at least 45 minutes of gentle heat to reach their full potential. Have a heavy-bottomed pan ready to distribute the heat evenly.
- large heavy-bottomed skillet
- baking sheet
- parchment paper
- sharp knife
What goes in.
- 3 lbyellow onions, peeled and sliced into uniform quarter-inch strips
- 3 tbspunsalted butter
- 1 tbspfresh thyme leaves
- 1 tspsea salt
- 1 sheetall-butter puff pastry, thawed
- 1 cupGruyère cheese, grated
- 1egg, lightly beaten for egg wash
Building Depth
Cook onions on low heat, stirring only when the bottom begins to brown. If the pan gets dry, add a splash of water to deglaze the sugars sticking to the metal.
The method.
Caramelize the onions
Melt butter in the skillet over medium-low heat. Add onions and salt. Cook, stirring occasionally, until they turn a deep, rich brown color—roughly 45 to 50 minutes.
Prepare the pastry
Preheat your oven to 400°F (200°C). Roll the puff pastry out on a sheet of parchment paper. Score a one-inch border around the edge with a knife, being careful not to cut all the way through.
Assemble the tart
Spread the cooled onion mixture inside the border of the pastry. Sprinkle the Gruyère cheese and thyme leaves evenly over the onions.
Bake
Brush the pastry border with the beaten egg. Bake on the center rack for 20 to 25 minutes, or until the pastry is puffed and golden brown and the cheese is bubbling.
Other turns to take.
Balsamic Finish
Add a tablespoon of balsamic vinegar during the last five minutes of onion cooking for a sharper acidity.
Anchovy Twist
Press two or three anchovy fillets into the onions before baking to add a deep, saline finish.
When it doesn't go to plan.
Use a mix of yellow and red onions if you want a slightly sweeter profile.
Ensure the onions are cool before spreading them onto the pastry to keep the butter in the dough from melting prematurely.
Place the baking sheet in the oven while it preheats; sliding the tart onto a hot tray ensures the bottom of the pastry crisps up immediately.
The ones that keep coming up.
Can I use store-bought pastry?
Yes, provided it is an all-butter variety; avoid versions made with palm or vegetable oils as they do not flake the same way.
How do I know when the onions are finished?
They should look like a dark jam and feel incredibly soft to the touch.
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