Place the fish in a shallow dish with four tablespoons of water, cover with microwave cling film. Cook on High for 3 minutes, stand for 5 minutes then drain, reserving the liquid in a measuring jug. Place the onion and green pepper into a microwave casserole dish along with the butter. Cook on High for 3 minutes. Stir in the flour and seasoning. Make up the fish liquid to 300ml with milk. Stir into the butter mixture. Cook on High for 2 minutes, stirring twice.
Mix the breadcrumbs and grated cheese. Sprinkle on the surface of the fish mixture. Cook on High for a further 2-3 minutes.