Preheat oven to 500°.
Arrange polenta slices in an 11 x 7-inch baking dish coated with cooking spray, allowing slices to overlap.
Heat a medium nonstick skillet coated with cooking spray over medium-high heat. Add mushrooms; cook 2 minutes, stirring frequently. Stir in garlic and salt. Cover, reduce heat, and cook 2 minutes. Stir in Alfredo sauce and basil.
Spoon mushroom mixture evenly over polenta. Top with cheese. Bake at 500° for 7 minutes or until thoroughly heated.