Preheat oven to 425 degrees F. Place broccoli in a steamer basket. Place basket in a large saucepan over 1 inch of boiling water. Steam, covered, for 6 to 8 minutes or until broccoli is crisp-tender.
Meanwhile, for sauce, in a medium saucepan heat 1 tablespoon butter over medium heat until melted. Add onion and garlic; cook until tender, stirring occasionally. Stir in flour, salt, and pepper. Gradually stir in milk. Cook and stir until thickened and bubbly. Gradually add cheese, stirring until melted.
Transfer broccoli to an ungreased 1 1/2-quart au gratin dish or 2-quart square baking dish. Pour sauce over broccoli. In a small bowl combine bread crumbs and 2 teaspoons melted butter; sprinkle over sauce. Bake, uncovered, about 15 minutes or until crumbs are light brown.