In medium saucepan, over low heat, heat milk, cheese and mustard until cheese melts and mixture is smooth. Stir in green onions, bacon and pepper; remove from heat. In large bowl, combine hot pasta and cheese mixture, tossing until well coated; spoon into greased 2 quart casserole. Cover; bake at 350F for 15-20 minutes. Uncover and stir; top with onion rings. Bake, uncovered, for 5 minutes more. Let stand 10 minutes before serving. Garnish as desired.