Heat oven to 375°F.
In 3-quart saucepan, cook onions in 2 tablespoons butter over medium heat 3 minutes. Stir in mustard. Add cheese, milk and pepper. Cook and stir until cheese melts and mixture is smooth. Stir in macaroni; spoon half into buttered 2 or 3-quart casserole. Arrange broccoli evenly over pasta, then top with remaining macaroni mixture. Melt remaining tablespoon of butter; toss with cracker crumbs in small bowl. Sprinkle crumbs over casserole. Bake for 25-30 minutes until bubbly and browned. Let stand 10 minutes before serving.