Rinse the fish, pat dry and season with salt, pepper and lemon juice. Cut the jacket potatoes into slices.
Peel and finely dice the onion and garlic. Sauté in fat over low heat, now add the defrosted spinach and sauté briefly. Season to taste with salt, pepper and nutmeg.
Now layer all the ingredients in a greased casserole dish as follows: First layer of potatoes (season them with salt and pepper, then the fish fillet and then the spinach. Finally, a layer of potato slices.
Mix the sour cream with the eggs, season with a little salt and pepper and pour over the casserole. Finally, sprinkle with the grated cheese.
Bake the casserole in the oven preheated to 180 degrees for about 30-40 minutes.