First soak the baccalà for 48 hours, changing the water every 4 hours or leave it for this long under a gently running tap. In a heavy shallow pan, fry the garlic and rosemary in the oil for a few seconds, add the diced tomato and fry briefly. Stir in the tomato purée, white wine and pepper to taste. Bring the mixture to the boil, then reduce the heat and add the baccalà. Simmer gently for 15–20 minutes, depending on the thickness of the fish.