Preheat oven to 375. In 3 qt. saucepan cook onions in 2 Tbsp. butter over med. Heat 3 minutes. Stir in mustard. Add milk, cheese & pepper. Cook & stir until cheese melts & mixture is smooth. Stir in macaroni, spoon half into buttered 2-3 quart casserole. Arrange broccoli evenly over pasta, then top with remaining macaroni mixture. Melt remaining butter, toss with cracker crumbs in small bowl. Sprinkle crumbs over casserole. Bake 24-30 minutes until bubbly and browned. Let stand 10 minutes before serving.