Butter bottom and sides of 4 individual casserole dishes or 13x" glass baking dish. In small bowl, combine 2 tbsp. butter and corn flake crumbs; set aside. In large sauté pan, melt 1/4 cup butter; stir in onions, sautéing until tender (about 5 min.). Whisk in flour until smooth; gradually whisk in half-and-half. Bring to a boil; continue cooking until thickened (about 1 min.), stirring constantly. Stir in salt, pepper and most of each cheese, reserving some cheese for the top. Cook pasta in salted water according to pkg. directions; rinse under cold water and drain completely. Stir pasta into cheese sauce; pour into prepared dish. Sprinkle remaining cheese and buttered corn flake crumbs over top. Bake in a preheated 350° oven until browned on top (20-25 min.).