Preheat oven to 350°.
Combine first 5 ingredients in a medium saucepan. Bring to a simmer over medium heat, and cook 10 minutes, stirring sauce occasionally. Spread 1/2 cup sauce in bottom of a 13 x 9-inch baking dish coated with cooking spray. Arrange the polenta slices over the sauce, and spread remaining sauce over polenta. Sprinkle with cheeses. Bake casserole at 350° for 20 minutes or until bubbly.