Polenta and Sausage Casserole

Prepare tomato-sausage sauce: In 5-quart Dutch oven over medium-high heat, cook sausages until browned, stirring with spoon to break them up. With slotted spoon, remove sausages to bowl. In same Dutch oven in hot olive oil, cook onion, celery, and carrot until browned. Stir in sausages and tomatoes with their purée over high heat, heat to boiling. Reduce heat to low; cover and simmer 10 minutes. Remove cover and simmer 10 minutes longer, stirring to break up tomatoes. Preheat oven to 350 degrees F. Prepare polenta: In 4-quart saucepan with wire whisk, mix cornmeal, chicken broth, and salt. Over medium-high heat, gradually add 4 1/4 cups boiling water, whisking constantly, until mixture thickens (about 5 minutes); whisk in Parmesan cheese. Grease 13" by 9" glass baking dish. Spread half of polenta in baking dish; top with half the tomato-sausage sauce, then half the shredded cheese. Repeat with remaining polenta and sauce. Bake casserole, uncovered, 15 minutes. Sprinkle with remaining cheese; bake about 20 minutes longer or until mixture is hot and bubbling. Let stand 15 minutes for easier serving.