Use a 250 ml (8 fl oz) ramekin to cut out 4 rounds from the puff pastry, making each one 1 cm (1/2 inch) larger all round than the ramekin. Place on a baking sheet and brush all over with beaten egg yolk. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 15–20 minutes until golden and crisp.
Meanwhile, heat the butter and oil in a saucepan. Add the leeks and a splash of water and cook for 5–7 minutes until soft. Stir in the flour, then pour over the stock and wine and leave to simmer for 7 minutes until the liquid has nearly boiled away. Add the prawns and cook for 3 minutes until cooked through, then stir in the cream and cook for a further 1–2 minutes until heated through. Season and add the lemon juice.
Divide the mixture between 4 x 250 ml (8 fl oz) ramekins. Top with a pastry lid and serve.