Thaw the spinach and squeeze it out or wash the fresh spinach and cut it into small pieces. Finely dice the onions and sauté in a pan in the oil until translucent. Add the spinach and season with salt, pepper and nutmeg.
Wash the salmon fillets, cut into cubes, drizzle with lemon juice and season with salt.
Melt the margarine in a saucepan, dust with flour and stir well. Deglaze with broth, stir in sour cream and season with salt, pepper and lemon zest.
Slice the potatoes and layer them in a baking dish. Cover with spinach, pour the salmon cubes over them and pour the sauce over them. Bake the casserole in the preheated oven on the middle rack at 175 ° C for about 25 minutes.