Wash the fish fillet and dry it with a cloth. Then cut the fish fillet into cubes and drizzle with the lemon juice. Clean the leek and cut into rings. Heat the butter in a pan. Add the leek and sauté.
Add the fillet cubes and season with salt and pepper. Cook covered for 5 minutes. Mix the sour cream and mustard and pour over the fish. Let everything stew for another 6 minutes.