Cut the fish into fairly large pieces and place in the bottom of an oven-proof casserole dish.
In a saucepan over medium heat, melt the butter. Stir in the flour and gradually blend in the milk. When the sauce is smooth, bring to the boil and stir until thickened. Remove from the heat and stir in the tomatoes. Pour sauce over the fish.
Bake in the preheated oven for 15 minutes then remove from the oven.
Increase the oven temperature to 190 C / Gas mark 5.
Mix together the breadcrumbs, cheese, parsley, salt and pepper in a small bowl. Sprinkle over the fish and return to the oven. Bake until golden, about 10 minutes. Garnish with lemon wedges and parsley.