1. Preheat the oven to 200°C. Melt the butter in a saucepan and stir in the flour; fry for a minute. Whisk the milk in gradually to prevent it from forming lumps. Simmer for about 5 minutes or until slightly thickened. Remove from the heat and stir in the mustard, nutmeg, salt and pepper to taste, as well as half of the cheese.
2. Arrange the fish and cauliflower in an ovenproof dish and pour the cheese sauce over the top. Arrange the tomato slices on top and sprinkle with the rest of the cheese. Bake for about 20 minutes or until the fish is done enough to flake. Serve with rice and a salad.