In 12-inch skillet, cook onion in 2 tbsp. butter. Add mushrooms and cook until tender. Stir in flour for 1 minute. Pour in 1 1/4 cups cream and simmer 2 minutes. Stir in squash, chicken and peas; season with salt and pepper. Set aside. Cook polenta in 3 1/2 cups boiling water with 1/2 tsp salt until thickened, 2 minutes. Stir in remaining 1/4 cup cream, 1 tbsp. butter and fontina until smooth. Serve chicken over polenta.