In a large saucepan, combine cornmeal and salt. Gradually add boiling water, whisking constantly. Cook and stir over medium heat for 5 minutes or until mixture comes to a boil. Remove from the heat. Stir in Parmesan cheese.
Spread polenta into a greased 13x9-in. baking dish. Bake, uncovered, at 350° for 20 minutes. Meanwhile, in a large skillet, cook sausage in oil over medium heat for 5 minutes or until no longer pink; drain. Add spaghetti sauce; cook for 5 minutes or until heated through.
Spread over polenta; sprinkle with mozzarella cheese. Bake 12-15 minutes longer or until cheese is melted. Let stand for 15 minutes before cutting.