For the pasta: In a 4-quart pot, cover the macaroni and salt with 1 inch of cold water. Bring to boil over medium heat. When the pasta comes to a boil begin testing for doneness. Cook to al dente. Drain in a colander and rinse with cold water to stop cooking
For the cheese sauce: While the pasta is cooking, melt the butter over medium heat in a 3-quart saucepan. Once the butter stops bubbling, whisk in the flour and cook until the mixture is a pale blonde, stirring occasionally, about 3 minutes. Add the mustard, paprika, onion and bay leaf and whisk to combine. Add the milk, whisking continually for 7 to 8 minutes or until slightly thickened. Remove from the heat and discard the bay leaf.
In a small bowl, lightly beat the egg. Add a few ounces of the milk mixture to the egg and whisk to combine. Add another few ounces of the milk mixture to the egg and thoroughly combine. Then add the egg mixture back to the milk mixture and whisk vigorously. Stir in three-quarters of the cheese and season with the salt and pepper. Fold the macaroni into the mix and pour into a 4-quart casserole dish.
For the topping: Top the macaroni with the remaining cheese. Toss the breadcrumbs with the melted butter to coat. Sprinkle the macaroni with the breadcrumb mixture in an even layer.
Bake for 30 minutes. Remove from the oven and rest for 5 minutes before serving.