Preheat the broiler. Set the oven rack about 3 inches from the heat In a large, heavy saucepan, bring the milk, sea salt, and nutmeg to a boil over high heat. (Watch it carefully, because milk will boil over quickly.) Add the polenta in a steady stream; stirring constantly with a wooden spoon, cook until it is thickened and leaves the side of the pan as it is stirred, about 2 minutes.
Remove the pan from the heat. Stir in the cream and half of the cheese, stirring to blend thoroughly. Pour into a 1-quart gratin dish, evening out the top with a spatula. Sprinkle with the remaining cheese.
Place the gratin dish on the oven rack and broil until the cheese is golden and bubbling, 1 to 2 minutes. Serve, with warm tomato sauce alongside.
In a large skillet, heat the oil, onions, garlic, and salt over moderate heat. Cook just until the onions are soft and translucent, 3 to 4 minutes. Place a food mill over the skillet and puree the tomatoes directly into the pan. Add the bouquet garni and stir to blend. Simmer, uncovered, until the sauce is thickened, about 15 minutes. Taste for seasoning. Remove and discard the bouquet garni. The sauce may be served immediately, stored in the refrigerator for up to 2 days, or frozen for up to 3 months.
Recipe courtesy of THE PARIS COOKBOOK (HarperCollins, 2001) by Patricia Wells