Melt the fat, add the flour and sauté while stirring. Add stock and milk while stirring, bring to the boil and simmer for about 5 minutes. Season to taste with lemon juice, salt and pepper. Clean and quarter the savoy cabbage and cut into strips. Wash, clean and slice the tomatoes. Wash salmon, dab dry and cut into cubes. Grease an oven dish (approx. 14 x 23 cm).
Put 2 tablespoons of béchamel sauce into the dish and spread. Place 3 sheets of lasagne on top and spread with 2 tablespoons of sauce. Spread 1/3 of the tomatoes, half of the savoy cabbage and half of the salmon one after the other. Do the same with the remaining ingredients. Finish with tomatoes. Sprinkle with cheese. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 30-40 minutes