Clean the cabbage, cut out the stalk. Cut cabbage into wide strips, wash and drain. Peel and chop onion. Melt 1 tablespoon of fat, fry onion in it until transparent.
Add cabbage and stew for about 5 minutes. Deglaze with stock, bring to the boil and simmer for 2 minutes. Drain the cabbage, catch the stock. For the béchamel sauce, melt 4 tablespoons of fat in a saucepan, add flour and sauté.
While stirring, add 500 ml stock and pour in milk. Bring to the boil, simmer for about 5 minutes. Season to taste with salt, pepper and lemon juice. Wash the salmon, dab dry and cut into cubes.
Grease an ovenproof dish (approx. 20 x 30 cm). Spread a little béchamel sauce thinly on the bottom of the dish and place 3 lasagne plates on top. Spread half of the savoy cabbage on top, place half of the salmon cubes on top and cover with béchamel sauce.
Repeat the procedure. Finish with lasagne plates and the rest of the sauce. Wash the lime and grate dry. Cut off 4 thin slices. Cut the rest of the lime into thin slices. Spread slices on the lasagne, cut cheese into slices and cover the lasagne.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes until golden brown. Arrange in portions and garnish with lime wedges and lemon balm.