Bring the measurement water to the boil in a large, heavy-based saucepan. Set aside 2 tablespoons of the polenta to use for the topping and put the remaining polenta in a jug. Pour into the water in a slow but steady stream, stirring vigorously with a wooden spoon to prevent any lumps forming. Reduce the heat to a slow simmer and cook, stirring frequently, for about 5 minutes, or until the polenta is thick and comes away from the side of the pan. Stir in the butter and season with salt and pepper.
Pour half the polenta into a greased baking dish, about 25 x 18 cm (10 x 7 inches). Top with the Gorgonzola and half the Parmesan. Cover with the remaining cooked polenta, then top with the tomatoes. Stir the remaining Parmesan into the reserved uncooked polenta for the topping and scatter over the dish.
Cook under a preheated medium grill for 4–5 minutes until the tomatoes are slightly softened and beginning to char. Serve steaming hot.