Preheat oven to 400 degrees F. Lightly grease a 3-quart rectangular baking dish; set aside.
In a large saucepan bring broth and butter to boiling. Meanwhile, stir together milk and polenta. Add polenta mixture to boiling broth. Cook and stir until bubbly; cook and stir for 3 to 5 minutes more or until very thick. Remove from heat. Stir in cream cheese, 3/4 cup of the mozzarella cheese, and 1/4 cup of the Parmesan cheese until well mixed. Spread two-thirds of the polenta mixture in the prepared baking dish; set aside.
In a large skillet cook sausage, mushrooms, onion, and garlic until meat is brown and onion is tender. Drain off fat. Stir pasta sauce into meat mixture; heat through. Spoon meat mixture over polenta in dish, spreading evenly. Spoon remaining polenta on sauce and sprinkle with remaining mozzarella and Parmesan cheeses.
Bake, uncovered, about 20 minutes or until heated through and top is lightly golden.