PREHEAT oven to 350°F. Cook and stir onion in 1 Tbsp. of the butter in medium saucepan on medium heat1 minute. Blend in flour. Gradually add milk, stirring until well blended; cook until thickened, stirring constantly. Add cheese; cook until melted, stirring frequently. Add corn and 1/4 cup of the crumbs; mix well.
ARRANGE broccoli in 12x8-inch baking dish; top with sauce. Melt remaining 1 Tbsp. butter; mix with remaining 1/4 cup cracker crumbs. Sprinkle over ingredients in baking dish.
BAKE 30 minutes or until heated through.