Preheat oven to 400°. Place half of broccoli in a steamer basket; place in a large saucepan over 1 in. of water. Bring to a boil; cover and steam 3-4 minutes or until crisp-tender. Place in a greased 13x9-in. baking dish; repeat with remaining broccoli., Meanwhile, in a small saucepan over medium heat, melt 4 tablespoons butter. Add onion; cook and stir until tender. Add garlic; cook 1 minute longer., Stir in flour until blended; gradually add milk. Bring to a boil; cook and stir 2 minutes or until thickened. Stir a small amount of hot mixture into egg yolk; return all to the pan, stirring constantly. Cook and stir 1 minute longer. Remove from heat; stir in the cheese, salt and pepper. Pour over broccoli. , In a small skillet, cook bread crumbs in remaining butter until golden brown; sprinkle over the top., Bake, uncovered, 15-18 minutes or until heated through.