- Prep
- 20 min
- Cook
- 55 min
- Total
- 75 min
- Serves
- 6
Ingredients
What you’ll need — tap to tick off as you go
- 900 g
- 40 g
- 1 tbsp
- 1 tsp
- 1 tsp
- ½ tsp
- 1 tsp
- 320 g
- 120 g
Process
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Melt the unsalted butter with the olive oil in a wide heavy pan over medium heat. Add the sliced yellow onions and the salt, stir to coat, then cook for 35 to 40 minutes, stirring every few minutes, until soft and deep golden brown.
If the onions are not browning after 25 minutes, sprinkle in the sugar. Stir in the fresh thyme and black pepper, cook 2 more minutes, then scrape onto a plate and let cool completely.
Heat the oven to 200°C. Line a baking tray with parchment paper and lightly butter it.
Roll the puff pastry on a floured surface to a 25 x 35 cm rectangle, about 3 mm thick, and transfer it to the prepared tray.
Score a 2 cm border around the pastry with a knife, prick the inner rectangle all over with a fork, then brush the border with the beaten egg.
Spread the cooled onions over the inner rectangle in an even layer, leaving the border bare, and scatter the grated Gruyère on top.
Bake for 20 to 25 minutes, until the pastry is puffed and deep golden at the edges and the cheese is melted and bubbling.
Cool on the tray for 5 minutes, then slide onto a board and cut into 6 rectangles with a sharp knife or pizza wheel.
Kitchen notes
Kitchen notes
- The one mistake most people make is rushing the onions: they need 35 to 40 minutes over medium heat to develop sweetness. If they start catching, lower the heat and add a splash of water rather than turning the heat up.
- Cool the onions fully before they go on the pastry. Warm topping melts the butter in the dough and the base turns soggy instead of crisp.
- Pricking the inner rectangle stops the centre from ballooning, so the onions and cheese sit on a flat base while the border puffs up as a rim.
- Grate the Gruyère coarsely; fine shreds release oil and turn greasy. A little grated cheese under the onions as well as on top adds a moisture barrier.
Substitutions
- Gruyère cheese→Comté or Emmental
- yellow onions→a mix of yellow and red onions for a sweeter, sharper flavour
- fresh thyme→1/2 tsp dried thyme
Storage & reheating
Keep leftovers in an airtight container in the fridge for up to 3 days. Reheat on a baking tray at 180°C for 8 to 10 minutes so the pastry crisps again; the microwave makes it limp. Freeze unbaked, assembled tart for up to 1 month, then bake from frozen at 200°C for 30 to 35 minutes.
FAQ
The questions we get asked most
Why is my onion tart soggy in the middle?
Either the onions were still warm when they went on the pastry, or the pastry was underbaked. Cool the onions completely and bake until the base is dry and golden, not just until the cheese melts. Pricking the inner rectangle also helps steam escape.
Can I caramelize the onions a day ahead?
Yes. Cook them fully, cool, and refrigerate in a covered container for up to 3 days. Bring them to room temperature or warm them briefly before spreading, because cold onions slow the pastry's rise and can leave the centre pale.
How do I stop the pastry from puffing up in the middle?
Prick the inner rectangle all over with a fork and leave a 2 cm border unscored. The border puffs into a rim while the pricked centre stays flat under the topping. If it still domes, press it down gently with a spatula halfway through baking.
Can I use frozen puff pastry?
Yes, and all-butter puff pastry gives the best flavour. Thaw it in the fridge overnight, not on the counter, and roll it while still cold. If it warms up and turns sticky, chill it for 10 minutes before topping.
What cheese works if I do not have Gruyère?
Comté and Emmental are the closest matches. A dry mozzarella or a mild cheddar also melts well, but avoid very soft cheeses like Brie, which release water and make the pastry soggy.
How do I know the tart is fully baked?
The border should be risen, crisp and deep golden, and the cheese should bubble rather than just look melted. Lift a corner with a spatula; if the underside is pale, bake 3 to 5 more minutes. It firms up as it cools for 5 minutes.
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