Make the polenta. During the last few minutes of cooking, stir in half the Fontina and the butter, then spread it, about 3/8 inch thick, on a clean counter or sheet pan to harden. Slice into rectangles or rounds--the exact size doesn't matter. This can be done hours in advance.
Heat the tomato sauce with half of the rosemary. Taste and season with salt, if needed, and a little pepper.
Preheat the oven to 400 degrees F. Spread 3/4 cup of the tomato sauce in a 3-quart gratin dish, then overlap the pieces of polenta with the remaining Fontina. Spoon the remaining sauce carefully in bands over the polenta, leaving the edges exposed. Crumble the Gorgonzola over the top and sprinkle with the rest of the rosemary and the parsley.
Bake, uncovered, for about 30 minutes or until the gratin is hot and bubbly. Garnish with the parsley, season with a little pepper, and serve.
Recipe courtesy of Deborah Madison, Vegetarian Cooking for Everyone, Broadway Books