Place a steamer basket in a large saucepan. Add water to just below the bottom of the steamer basket. Bring water to boiling. Add broccoli to steamer basket. Cover and reduce heat. Steam for 6 to 8 minutes or until just tender.
Meanwhile, for sauce, in a medium saucepan cook and stir the onion and garlic in the 1 tablespoon hot butter until tender. Stir in flour, salt, and pepper. Stir in the milk. Cook and stir until thickened and bubbly. Gradually add cheese, stirring until melted.
Transfer broccoli to a 1 1/2-quart au gratin dish or 2-quart square baking dish. Pour sauce over broccoli. Combine bread crumbs and the 2 teaspoons melted butter; sprinkle over sauce. Bake, uncovered, in a 425 degree F oven for 15 minutes or until crumbs are lightly browned. Makes 6 servings.