Melt butter in a large skillet over medium heat; add squash and next 3 ingredients, and sauté 10 minutes or until vegetables are tender.
Stir in water chestnuts, Swiss Cheese Cream Sauce, and, if desired, diced pimientos. Spoon squash mixture into a lightly greased 13- x 9-inch baking dish.
Bake at 350º for 15 minutes. Sprinkle evenly with Buttery Parmesan Topping; bake 30 to 35 more minutes or until set and golden brown. Let stand 10 minutes before serving.