Melt the butter and sear the scallops quickly on each side, they must not brown. Remove from the heat.
Add the chopped shallots without browning them. Sprinkle over a tbsp. of flour. Moisten with the white wine, then with the fish stock diluted in the water. Bring to the boil and then reduce by half, then add the scallops, the chopped mushrooms and the prawns.
Leave to cook for 4 to 5 minutes. Add the salt, pepper and the nutmeg. Take note: the fish stock is already quite salty.
Off the heat, quickly add the egg yolk and the cream. Set aside off the heat.
Spread out and cut up the puff pastry into four squares of approximately 15cm x 15cm each. Place them on a sheet of greaseproof paper. Prick the pastry with a fork. Place the garnish on top and sprinkle with the pre-grated Beaufort Close up the four corners like an envelope.
Place in a hot oven at 200°C for around 20 minutes, until the pastry turns golden brown. Serve immediately.