Preheat oven to 375° F. Cook macaroni according to package instructions, or until al dente. Drain, rinse and set aside.
In large sauce pan, sauté onion in melted butter for two minutes or until transparent. Blend in flour, stirring well to incorporate, and cook for an additional minute, stirring. Gradually whisk milk into butter and flour mixture. Cook, stirring constantly, until mixture comes to a boil and thickens. Over low heat, add salt, pepper, hot sauce and mustard. Stir in 2 1/2 cups cheese.
Add cooked macaroni to cheese sauce and mix well. Place macaroni mixture in buttered, oven proof, 9” x 9” dish. Top with remaining 1/2 cup cheese and bread crumbs. Bake for 25 minutes or until top is golden brown.