Place half of broccoli florets in an 11- x 7-inch baking dish. Add 1/4 cup water, and microwave at HIGH 7 minutes or until tender; drain. Repeat with remaining half of broccoli and 1/4 cup water. Set aside.
Melt butter in a large skillet over medium heat; sauté onion 7 minutes or until tender.
Combine broccoli, onion, Cheddar Cheese Cream Sauce, sliced water chestnuts, salt, and, if desired, pimientos. Spoon mixture into a lightly greased 13- x 9-inch baking dish.
Bake at 350º for 15 minutes. Sprinkle evenly with Cheddar cheese; bake 30 to 35 more minutes or until set. Let stand 10 minutes before serving.