If the prawns are frozen allow to defrost. Put the potatoes into a pan of boiling salted water, cook steadily for 10 minutes only then strain. Wash and trim the spinach, crush the garlic; slice the part-boiled potatoes. Melt the butter or margarine in a large pan, add the garlic, spinach and nutmeg and a little seasoning. Cook for 2 minutes then spoon into the base of a large shallow dish; top with the prawns. Preheat the oven to 190C/375F,Gas Mark 5. For the sauce: Melt the butter or margarine in a saucepan, add the flour and cook for 1 minute. Add the milk and bring to the boil, stirring or whisking briskly. Cook for 2 to 3 minutes. Remove from the heat, add halfthe cheese and season to taste. Pour the sauce over the prawns. Top with the sliced potatoes and the remaining cheese. Bake for 20 minutes or until golden brown. Serve with tomatoes and a green salad. In a microwave: Make the sauce in a basin.