Ingredients

• 5 large day-old butter croissants (about 350 g)day-old, or torn and dried in a low oven; fresh turns to paste
• 300 g chestnut mushroomssliced
• 2 medium onionsthinly sliced
• 25 g butter
• 15 ml olive oil
• 2 tsp thyme leaves
• 1 small garlic clovegrated
• 5 large eggs
• 350 ml whole milk
• 100 ml double cream
• 1 tsp Dijon mustard
• 1/2 tsp salt
• 1/2 tsp freshly ground black pepper
• 160 g Gruyeremelts more smoothly than cheddar and is worth the difference here

Method

1. 01

   Caramelise the onions

   Melt the butter with the olive oil in a large frying pan over medium-low heat, add the sliced onions and a pinch of salt and cook for 15-20 minutes, stirring every few minutes, until deep golden and jammy. If they are colouring in under 10 minutes the heat is too high. You want them sweet, not scorched.

   25 g butter · 15 ml olive oil · 2 medium onions

   Patience with the onionsTwenty minutes on medium-low is not ten on medium. Rushed onions stay harsh.

2. 02

   Brown the mushrooms hard

   Raise the heat to medium-high, add the sliced mushrooms and cook for 6-8 minutes without salt, in two batches if the pan looks crowded. They will release water, the pan will go quiet as it evaporates, and only then will they brown. Add the grated garlic clove and 2 tsp thyme for the final minute.

   300 g chestnut mushrooms

   Brown the mushrooms hardCrowded, they stew grey and watery, and that water thins the custard. Two batches if needed.

3. 03

   Assemble

   Tear the croissants into large pieces and arrange them in a buttered 23 x 33 cm dish, tucking the onion and mushroom mixture and about two thirds of the 160 g Gruyere between them. Leave some croissant tips standing proud. They crisp while the rest turns soft and silky.

   5 large day-old butter croissants (about 350 g) · 160 g Gruyere

4. 04

   Pour the egg mixture

   Whisk the eggs, milk, cream, 1 tsp Dijon, salt and black pepper until completely smooth, then pour evenly over the dish and press the croissants down gently.

   5 large eggs · 350 ml whole milk · 100 ml double cream · 1/2 tsp salt · 1/2 tsp freshly ground black pepper

5. 05

   Let it soak

   Rest for 20 minutes at room temperature, pressing once more halfway through. The croissant should feel heavy and saturated rather than springy.

6. 06

   Bake

   Heat the oven to 180 C / 160 C fan. Scatter over the remaining Gruyere and bake for 30-35 minutes, until puffed, browned and just set. A knife in the centre should come out with a soft set clinging to it rather than raw liquid. Overbaked, it goes from silky to rubbery quickly.

   Stop while the centre just setsIt carries on cooking in the dish. Silky and rubbery are four minutes apart.

7. 07

   Rest before serving

   Rest for 10 minutes so the centre finishes setting and the bake holds its shape when you cut it.