From "The Green's Cook Book: Extraordinary Vegetarian Cuisine from the Celebrated Restaurant, by Deborah Madison with Edward Espe Brown". Fabulous. I recommend taking the trouble to make the tomato sauce yourself instead of using bottled sauce; also, try to find fontina cheese, it's very good in this recipe. If you can't find fontina, look for a semi-soft flavorful but not sharp cheese.
Make the polenta and sauce the day ahead of time. Then you can assemble and bake the dish very quickly.
Make a simple tomato sauce. Warm the olive oil with the bay leaf, basil, parsley, onion and garlic, and cook slowly for 3 to 4 minutes. Add the tomatoes, raise the heat to medium high, and cook until they are quite soft and broken apart, about 15 minutes. Pass them through a food mill (I used a food processor --mrm); then return the sauce to the pan, and simmer until quite thick. If needed, add a pinch or two of sugar to balance the acidity, and season with salt and freshly ground black pepper. This should make 1 1/2 cups.
While the sauce is cooking, bring 4 1/2 cups water to a boil and add 1 1/2 teaspoons salt. Whisk in the cornmeal in a stream so that lumps don't form, and when it has all been added, switch to a wooden spoon, lower the heat, and stir for 15 minutes. Pour it into a loaf pan or 9-by-12 baking dish, and set it aside to cool. When it is thoroughly cool and firm, turn it out and slice into pieces about 1/2 inch thick about 3 inches long. If a loaf pan was used, cut the pieces again, diagonally. (Note from mrm: just cut the polenta any way you like - I experimented with making 3-inch-long diagonal cuts and then making an interesting geometric pattern when I laid it out.)
Preheat the oven to 400 F. Lightly butter a gratin dish (I used a 9-by-12 dish), and spread about 1 cup of the tomato sauce over the bottom. Arrange the polenta and the slices of Fontina in overlapping layers. Carefully spoon the remaining tomato sauce over the layers in a decorative way, leaving bands of red sauce between the yellow polenta and cheese. Crumble Gorgonzola over the top, pepper generously (or not, as you like --mrm), and bake the gratin, uncovered, for 25 to 35 minutes. The cheese will melt into the tomato, making a sauce to spoon over the polenta. Garnish with the fresh herbs.