Prepare tomato-sausage sauce. In a 5-quart dutch oven, cook sweet and spicey meat over medium-high heat, breaking up meat with side of spoon, until browned. With slotted spoon, transfer meat to bowl. Discard fat from dutch oven.
Add oil to dutch oven. Add onion, celery, and carrot and cook over medium high heat until browned. Stir in sausage and tomatoes with thier puree, breaking up tomatoes with side of spoon. Heat to boiling over high heat. Reduce heat; cover and simmer 10 minutes. Remove cover and simmer 10 minutes longer.
Preheat oven to 350 degrees. Prepare polenta; In a 4-quart saucepan with wire whisk, mix cornmeal, broth, and salt. Over medium high heat, add boiling water and cook, whisking constantly, until mixture has thickened, about 5 minutes. Whisk in parmesan.
Grease 13x9 baking dish. Evenly spread half of polenta mixture in baking dish; top with half of tomato sausage sauce, then half of fontina and 1/2 of the mozzerella. Repeat with remaining polenta mixture and sauce.
Bake casserole 15 minutes. Sprinkle with the remaining Fontina and Mozzerella; bake until mixture is bubbling and cheese is golden, about 20 minutes longer. Let stand 15 minutes for easier serving.