Note: We cut the amount of polenta and water in half and still had plenty to make two meals.
In a saucepan, bring water to a boil. Dribble in the cornmeal in a steady stream, whisking constantly. Add salt. When polenta begins to boil, reduce heat the low. Cook, stirring frequently, until it is thick and pulls away from the sides (30-40 minutes). Oil two 8 inch baking pans. Pour polenta into the prepared pans. Cool until firm; cover and refrigerate.
To make the sauce, in a large saucepan over medium heat, melt the butter with the olive oil. Add the onion, carrot, and celery and cook, stirring, until tender but not browned, about 10 minutes. Stir in the garlic. Add the meat and cook, stirring to break up any lumps, until browned, about 10 minutes. Add the wine and cook until the liquid evaporates. Add the tomatoes and their juice and simmer, uncovered until thickened (30-45 minutes).
Preheat oven to 400°. Oil a large rectangular baking dish.
Cut the polenta into 4 inch squares and arrange half of the squares in the bottom of the baking dish. Spoon on half of the meat sauce, covering evenly, and top with half of the mozzarella slices. Repeat the layers using the remaining ingredients. Sprinkle with grated cheese.
Bake until the cheese melts and the sauce is bubbling, about 40 minutes. Let stand for five minutes before serving. Makes 4 very large servings.
From Williams-Sonoma, Savoring Italy, Michele Scicolone, Time-Life, 1999.