Thaw the salmon and cut into cubes. Peel the onion, cut into small, fine cubes. Remove the outer leaves from the cabbage, wash, quarter and remove the stalk. Cut the savoy cabbage into small strips. Peel the potatoes, cut into cubes and wash.
Heat some of the butter in a saucepan, sauté the onion cubes, the potato cubes and the savoy cabbage strips in it. Pour in the broth, simmer in a closed saucepan over low heat.
Grease a small baking dish and add the salmon cubes. Mix with the zest of lemon, pepper, salt and sesame seeds and fry in the oven for about 8 minutes.
Melt the rest of the butter in a saucepan, add flour and make a roux, add some of the soup liquid and stir well, making sure that no lumps are formed. Add the whole thing to the soup, pour on the cream and stir everything well. Season to taste with salt, pepper and nutmeg. Put the soup on preheated plates and garnish with the salmon cubes.