Preheat oven to 350°. In a small skillet, saute water chestnuts, onion and mushrooms in 2 tablespoons butter until onion is crisp-tender, 4-5 minutes; set aside. , In large skillet, melt remaining 4 tablespoons butter; stir in flour until smooth. Stir in milk, broth, soy sauce, pepper sauce and salt. Bring to a boil; cook and stir until thickened, about 2 minutes. Remove from heat; stir in green beans and cheese. , Spoon half of the bean mixture into a greased 1-1/2-qt. baking dish. Layer with water chestnut mixture and remaining bean mixture. , Bake, uncovered, 45 minutes. Top with french-fried onions. Bake until heated through, about 5 minutes longer.