Remove stems from shiitake mushrooms, and thinly slice.
Sauté onion in a large nonstick skillet coated with cooking spray over high heat 3 minutes. Add shiitake and button mushrooms, and sauté 3 to 4 minutes or until onion is tender. Add 3/4 cup milk, and bring to simmer.
Whisk together cornstarch and 2 tablespoons milk; stir into mushroom mixture. Add cream cheese and wine, stirring until blended. Remove from heat. Stir in salt, pepper, and nutmeg.
Spread 3 tablespoons of Cream Sauce in a 13- x 9-inch baking dish coated with cooking spray. Layer one-third of polenta slices over sauce. Sprinkle 1/4 cup Parmesan cheese over polenta. Top with mushroom mixture. Top with one-third polenta slices and remaining Cream Sauce; top with remaining one-third of polenta slices.
Bake at 375° for 35 to 40 minutes or until bubbly. Sprinkle with remaining 1/4 cup Parmesan. Broil at high 3 to 5 minutes or until cheese is lightly browned. Remove from heat. Cover loosely with aluminum foil, and let stand 15 minutes before serving.
Note: For testing purposes only, we used Marjon Original Polenta found in the produce section of the supermarket.