Preheat oven to 350 degrees F. Grease a 3-quart rectangular baking dish; set aside. In a large saucepan cook macaroni according to package directions; drain. Return to saucepan. Stir in cheddar and American cheeses; set aside.
In a medium skillet cook sweet onion and garlic in butter until tender. Stir in green onions and sweet peppers. Cook and stir 1 minute more; stir onion mixture into pasta-cheese mixture. Spoon into prepared baking dish; set aside.
In a medium bowl whisk together milk, eggs, flour, mustard, hot pepper sauce, paprika, and salt until smooth; pour evenly over pasta mixture in baking dish. Bake, covered, 20 minutes. Uncover; bake 15 to 20 minutes more or until bubbly and heated through. Let stand 5 minutes. Makes 6 servings.