Wash the fish, dab dry and cut into cubes. Sprinkle with lemon juice. Clean and wash the zucchini and tomatoes. Peel onions. Cut zucchini into slices, onions into wedges. Cut tomatoes in half, remove seeds and dice.
Grate cheese finely. Brown onions lightly in 20 grams of hot fat. Add zucchini and fry. Season with salt and pepper. Heat 20 grams of fat in the pot. Sauté the flour in it. Deglaze with stock and milk and bring to the boil. Simmer for five minutes. Stir mustard and cheese into the sauce. Add the fish and vegetables to the sauce and simmer for ten minutes, stirring occasionally. Season to taste with salt and pepper if necessary.
Sauté the flour in it. Deglaze with stock and milk and bring to the boil. Simmer for five minutes. Stir mustard and cheese into the sauce. Add the fish and vegetables to the sauce and simmer for ten minutes, stirring occasionally. Season to taste with salt and pepper if necessary. Serve the fish ragout in a preheated tureen. Serve with rice or parsley potatoes