Peel and finely dice the onion and garlic. Sauté both in 1 tbsp. hot butter. Add spinach and steam for about 10 minutes. Season to taste with salt, pepper and nutmeg. Drain spinach on a sieve and squeeze out a little.
Grate cheese. Sweat flour in 3 tbsp. hot fat. Stir in milk, bring to the boil and simmer for about 5 minutes. Season to taste with salt, pepper and nutmeg. Melt about 2/3 cheese in it.
Wash the salmon, dab dry and cut into strips. Season with salt and lime juice.
Grease an ovenproof dish (16 x 24 cm; approx. 2 l capacity). First spread some sauce in it and place 3 lasagne plates on top. Cover with half spinach and half salmon. Place 1/3 of the sauce and 3 lasagne dishes on top.
Layer the remaining ingredients in the same way. Finish with the rest of the sauce and cheese. Bake salmon-spinach lasagne in a preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 40 minutes until golden brown.