For the salmon gratin, wash the potatoes and cook in water for about 20 minutes. Quench, peel and let cool
Peel and finely dice the onions and garlic. Heat 1 tablespoon of fat in a pot. Fry garlic and half of the onions in it. Add spinach without defrosting, pour on 100 ml water. Cover and defrost over medium heat for about 15 minutes. Drain if necessary. Season with salt and pepper
Salmon wash, dice. Sprinkle with lemon juice and let it steep a little. Cut potatoes into slices
Heat 2 tablespoons of fat in a pot, sauté the rest of the onions. Sauté the flour in it. Stir in milk, except for 5 tbsp. Bring to the boil and simmer for about 5 minutes. Melt 75 g cheese in the sauce. Season to taste with salt, pepper and nutmeg. Whisk 5 tbsp. milk and eggs and stir into the sauce (do not boil any more!)
Layer potatoes, spinach and salmon in a greased casserole dish (approx. 30 cm long). Pour sauce over it, sprinkle with the rest of the cheese. Bake the salmon au gratin in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for 30-40 minutes, garnish