Cook macaroni according to package directions. Drain; place in large bowl. Cover with plastic wrap.
Heat oven to 400 degrees F. Grease 13 x 9 x 2-inch baking dish.
Meanwhile, heat oil in medium-size saucepan. Add onion; cook until very tender, 10 minutes. Stir in flour and salt; cook 1 minute. Whisk in milk, stirring, until sauce comes to boiling; cook 1 minute. Remove from heat. Whisk in cheese, mustard and pepper sauce until smooth. Add mixture to macaroni; toss to blend. Spoon into prepared baking dish.
Combine bread crumbs, butter and parsley in small bowl. Sprinkle over macaroni and cheese.
Bake in 400 degree F oven 20 to 25 minutes, until top is golden.