Preheat the oven to gas 6, 200°C, fan 180°C. In a small pan, melt the butter. Stir in the flour to make a thick paste, then, off the heat, gradually whisk in the milk. Return to the heat and cook the sauce over a low heat for a few minutes, stirring regularly, until it’s thickened. Season and stir in the parsley and cheese.
Place the fish in a gratin dish, season and pour over the sauce. Scatter the tomatoes around. Cook for 10 minutes, or until the fish is just cooked through, then place the dish under the grill for a few minutes more, until the sauce is golden and bubbling. Serve with new potatoes and veg.