Polenta, seasoned with fresh basil and garlic, serves as the crust in this elegant make-ahead appetizer tart. Make and chill the polenta up to one day ahead. Just before baking, top with portobello mushrooms, tomato sauce, and Parmesan cheese.
Coat 8-inch tart pan with removable bottom or 8-inch springform pan with cooking spray. Bring water, salt to boil in saucepan. Gradually stir in cornmeal; cook, stirring, 3 minutes, until thickened. Remove from heat. Stir in 1/2 cup Parmesan, basil, garlic, pepper. Spread in pan, leaving 1/2-inch rim all around. Cover; refrigerate 1 hour, until firm. (Can be prepared a day ahead.)
Heat oven to 350 degrees F. Sauté mushrooms in oil in large skillet over medium-high heat 5 minutes.
Top polenta with sauce, mushrooms, remaining Parmesan.
Bake in 350 degree F oven 15 minutes. Makes 8 servings.