Melt the fat in a saucepan and sauté the flour while stirring. Gradually add stock and milk. Bring sauce to the boil, stir in 40 g of grated cheese. Season the sauce to taste with salt, pepper and nutmeg and place in a shallow casserole dish (approx. 1.5 litres capacity).
Put peas in boiling salted water, boil for 2 minutes, drain and rinse in cold water, drain. Cut croissants horizontally, but do not cut through. Fill each croissant one after the other with a slice of processed cheese, salmon and about 2 tablespoons of peas.
Place the croissants in the casserole dish and sprinkle with the remaining cheese. Bake in the preheated oven (electric oven: 200 °C/ convection oven: 175 °C/ gas oven: level 3) for 10-15 minutes. Garnish the casserole with dill.