Trim, clean and fillet the mackerel. Dip each fillet in seasoned flour. Melt the margarine in the bottom of the grill pan, add the fillets, brush them with the melted fat and grill for 8–10 min., turning once. Cut the tomatoes in half and grill at the same time, for garnish. Meanwhile prepare the gooseberry sauce. Serve the fillets on a hot dish, garnished with the tomatoes and serve the sauce separately.