Ingredients

• 320 g all-butter puff pastryall-butter, ready-rolled
• 2 onionsthinly sliced
• 300 g chestnut mushroomsfirm and dry
• 25 g butter
• 1 tbsp olive oil
• 1 tsp brown sugar
• 1 tsp balsamic vinegar
• 120 g Gruyeregrated
• 1 tsp Dijon mustard
• 1 eggbeaten
• 6 thyme sprigs
• 3/4 tsp fine salt and freshly ground black pepper

Method

1. 01

   Caramelise the onions

   Melt half the 25 g butter in a frying pan over low-medium heat, add the sliced onions and a good pinch of salt, and cook for 20 minutes, stirring every few minutes. Add the brown sugar and 1 tsp balsamic and cook for a further 3-5 minutes until jammy and deep brown. Spread on a plate to cool.

   25 g butter · 2 onions · 1 tsp brown sugar

2. 02

   Brown the mushrooms hard and dry

   Heat a separate wide pan over high heat for 2 minutes. Add the olive oil and the mushrooms in a single layer and do not salt them. Leave for 2-3 minutes before stirring, then cook for 6-8 minutes until all the water has boiled off and the edges are dark. Add the remaining butter, the thyme leaves and pepper for the final minute. Cook them in two batches if your pan is not wide enough. This is the step that decides whether the pastry stays crisp.

   1 tbsp olive oil · 300 g chestnut mushrooms

   Moisture is the enemyAny water left in the mushrooms or onions ends up in the pastry. Cook until the pan is dry, then cool.

3. 03

   Prepare the pastry

   Heat the oven to 200 C / 180 C fan. Lay the pastry on a lined tray and score a 2 cm border all the way round without cutting through. Prick the inner rectangle all over with a fork and spread it with the 1 tsp Dijon.

   320 g all-butter puff pastry

4. 04

   Layer the filling

   Scatter half the 120 g Gruyere over the inner rectangle. The cheese underneath acts as a seal against any remaining moisture, then the cooled onions, then the mushrooms, then the rest of the cheese. Brush the border with the beaten egg.

   120 g Gruyere

   Cheese as a barrierThe layer under the vegetables is not only flavour. It keeps the base crisp.

5. 05

   Bake

   Bake for 20-25 minutes until the border is deeply risen and bronzed and the base is dry and crisp when you lift a corner with a palette knife. If the top is browning but the base is pale, give it 5 more minutes on a lower shelf.

   Check the base, and reheat in the ovenLift a corner: dry and layered, not pale. A microwave makes puff leathery in seconds.

6. 06

   Rest and slice

   Rest for 5 minutes before slicing so the cheese sets enough to cut cleanly.