- Preheat oven to 350 °F.
- In a large pot of boiling, salted water, cook the pasta until it is al dente.
- While the pasta is cooking, melt the butter in a separate pot.
- Whisk in the flour and mustard and keep it moving for about five minutes. Make sure it is free of lumps.
- Stir in the milk, onion, bay leaf, and paprika. Simmer for ten minutes and remove the bay leaf.
- Temper in the egg.
- Stir in ¾ of the cheese.
- Season with salt and pepper.
- Fold the macaroni into the mix and pour into a 2-quart casserole dish.
- Top with remaining cheese.
- Melt the butter in a sauté pan, add breadcrumbs, and toss to coat.
- Top the macaroni with the bread crumbs.
- Bake for 30 minutes.
- Remove from oven and rest for 10 minutes before serving.