Cut out four pieces of foil large enough to enclose the halibut steaks. Brush the foil with olive oil. Sprinkle the halibut steaks on each side with salt and pepper and lay them on the foil. Scatter the shallots over the fish, then arrange the tomatoes over the shallots. Sprinkle with the chopped rosemary. Dribble a little olive oil over each steak and wrap up the foil.
Put the foil parcels on a rack in a roasting pan and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes. To serve, garnish the halibut steaks with sprigs of rosemary. Roughly mashed parsleyed new potatoes and a crisp green salad are a good accompaniment.