Heat 3 tablespoons of olive oil in a large (2-3 qt) saucepan on medium heat. Add the sliced onion and sausage (removed from casing) and sauté, stirring often, until the meat is browned. Add the garlic, salt, pepper, and fennel seeds(optional) to the sauté pan. Cook, stirring occasionally, for 2 minutes. Add the tomato, cover the pan, lower the heat to medium, and simmer for 20 minutes.

While the sauce is cooking, coat the bottom of a 9x13-inch lasagna or gratin pan (a pan that can handle the heat of a broiler) with a 1 tablespoon oil. Add the cooked polenta (or slices) to the pan. Coat the top of the polenta with a little more olive oil. Broil the polenta about 4 inches from the heating element until golden brown and crispy, about 10-15 minutes.

Pour sauce over broiled polenta, then arrange mozzarella slices over the top and return the casserole to the broiler. Broil until the cheese is melted and beginning to brown, about 2 minutes. Let cool and set for a 5 or 10 minutes before serving.

Note: To spice this up use hot Italian sausage and red pepper flakes.