Savoy cabbage clean, wash, quarter and cut into strips from the stalk. Blanch in boiling salted water for about 2 minutes. Drain well. Wash salmon and pat dry, cut into pieces. Peel and finely chop the onion.
For the sauce, heat butter in a pot. Fry the onion until transparent. Dust with flour and sauté until light brown. Stir in milk, cream and stock. Bring to the boil and simmer for about 5 minutes. Season to taste with salt, pepper and lemon juice.
Put some sauce in a greased large casserole dish. Place 3 lasagne dishes, some sauce, half the savoy cabbage and again 3 lasagne dishes. Spread some sauce on top. Season salmon with salt and pepper and place on top. Spread 3 lasagne dishes, some sauce and the rest of the savoy cabbage on top. Finish with the remaining lasagne and the rest of the sauce. Sprinkle with cheese. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for about 40 minutes.