Peel and chop the onions. Heat 1 tablespoon butter. Sauté half of the onions in it. Add spinach and 100 ml water, defrost for about 15 minutes. Season with salt, pepper and nutmeg.
Sauté the flour in 4 tbsp. hot butter. Stir in milk, bring to the boil and simmer for about 5 minutes. Season to taste. Wash salmon, dab dry. Season with lemon, salt and pepper.
Spread a high casserole dish (14 x 23 cm) on the bottom with some sauce. Pile in 3-4 lasagne dishes, half spinach and 3-4 lasagne dishes. Spread with sauce. Pile on salmon, 3-4 lasagne dishes, the rest spinach, some sauce and the rest of the lasagne dishes.
Cover with the rest of the sauce and sprinkle with cheese. Bake in a hot oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for 40-45 minutes.
Wash and halve the tomatoes. Shrimps wash and dab dry. Fry briefly on each side in 1 tbsp. hot butter until pink. Season and take out. Sauté the remaining onions and tomatoes in the frying fat.
Arrange lasagne with prawns, tomatoes and onions.