Melt 3 Tbsp. butter on low heat, add onion & cook 3-5 min. until tender. Stir in flour & cook 2 minutes, stirring constantly. Blend in milk, cook 3-5 min. or until thickens & bubbly, stirring constantly. Add cheese, salt & pepper. Remove from heat & stir until cheese melts. Spoon over fish. Melt 3 Tbsp. butter over medium heat, remove from heat. Stir in crumbs, spread on fish. Bake at 350 degrees for 40-45 minutes. Let stand 10 min. to serve. Serves 4-6.