Cook and stir onions in 1 Tbsp. butter in medium saucepan on medium heat 1 min. Blend in flour. Gradually stir in milk; cook and stir 2 to 3 min. or until thickened.
Add cheese; cook 2 to 3 min. or until melted, stirring constantly. Stir in corn and 1/4 cup cracker crumbs.
Place broccoli in 12x8-inch baking dish; top with sauce. Melt remaining butter; mix with remaining cracker crumbs.