Mary Berry's mushroom and spinach en croûte recipe

Ingredients

• 1 tbsp olive oil
• 0.5 large onion, chopped
• 1 garlic clove, crushed
• 150 g chestnut mushrooms, sliced
• 225 g baby spinach
• 125 g ricotta
• 75 g Gruyère, grated
• 1 egg yolk
• 1 dash of Tabasco
• 1 pinch each of salt and freshly ground black pepper
• 1 pinch plain flour, to dust
• 0.5 x 500g/17.5oz packet ready-rolled all-butter puff pastry (freeze the other half for another occasion)
• 1 egg beaten with 1 tbsp milk