Cook and stir onions in 2 Tbsp. butter in medium saucepan on medium heat 1 min. Stir in flour; cook 1 min., stirring frequently. Gradually whisk in milk until blended. Cook 10 min. or until thickened, stirring constantly. Add cheese; cook and stir 3 to 5 min. or until melted. Add broccoli, corn and 1/4 cup cracker crumbs; mix well.