Polenta Lasagna with Creamy Mushroom Sauce

Name: Polenta Lasagna with Creamy Mushroom Sauce
Ingredients: 1 medium-size onion, halved and thinly sliced crosswise (1 cup); 8 oz. button mushrooms, sliced; 8 oz. shiitake mushrooms, stems removed and caps thinly sliced; 2 cloves garlic, minced (about 2 tsp.); 3/4 cup plus 2 Tbs. low-fat milk; 2 Tbs. cornstarch; 1/4 cup reduced-fat cream cheese, softened; 1/4 cup dry white wine or low-sodium vegetable broth; 3/4 tsp. salt; 1/4 tsp. freshly ground pepper; 1/4 tsp. ground nutmeg; 2 17-oz. tubes polenta, cut into 1/2-inch slices; 1/2 cup grated Parmesan cheese