In a covered saucepan, cook onions in a small amount of water for 5 minutes.
Add broccoli; cook 5 more minutes.
Drain.
Place in a 2-qt. baking dish. In a small saucepan, melt 4 tablespoons butter; stir in flour, salt and pepper. Gradually add milk; cook and stir until thickened.
Add cream cheese; stir until melted.
Pour sauce over vegetables; sprinkle with cheddar cheese. Cover and bake at 350° for 30 minutes. Melt remaining butter; combine with crumbs.
Sprinkle over casserole. Return to the oven, uncovered, for 15 minutes.