Classic Wisconsin Macaroni and Cheese

12 ounces elbow macaroni
3 tablespoons butter
1/4 cup diced white onion
2 tablespoons flour
2 cups milk
1 teaspoon salt
1/2 teaspoon pepper
1/2 teaspoon hot pepper sauce
1 tablespoon prepared mustard
3 cups (12 ounces) sharp Wisconsin Cheddar cheese, shredded
3 cups coarse bread crumbs

Preheat oven to 375 degrees F. Cook macaroni according to package instructions, or until al dente. Drain, rinse, and set aside.

In large saucepan, saut