Preheat oven to 350 degrees.

Bring a large pot of salted water to a boil. Cook macaroni to almost al dente (6 minutes or so). Drain and rinse briefly with cold water.

In a small pot, melt the butter until it stops bubbling (a minute or two). Whisk in flour and cook for 2-3 minutes to achieve a blonde color. Make sure it is free of lumps.

Add mustard, paprika, bay leaf, and onion. Cook for a minute or two until fragment. Add milk and simmer for 10 minutes. Remove bay leaf.

Temper in the egg*. Add salt and pepper. Stir in 3/4 of the cheese. Fold the macaroni into the mix and pour into a 2 quart casserole dish(round is best). Top with remaining cheese.

Melt butter in saute pan or microwave safe cup. Add breadcrumbs and mix to coat. Top macaroni with bread crumbs. Bake for 30 minutes. Remove from oven and rest for 5 minutes before serving.

* In order to keep the egg from scrambling when added to the hot milk mixture, add a small amount of the hot milk mixture to the egg first and whisk to raise temperature of the egg. After doing this a couple of times, add the egg into the milk mixture whisking constantly to incorporate it.