Adapted from an Alton Brown recipe.
1/2 pound cavatappi (preferred) or elbow macaroni (see note)
3 TB butter
3 TB flour
2 tsp powdered mustard (Coleman's preferred)
2-1/2 c milk, warmed
1/2 c diced onion
2 bay leaves
1 tsp paprika (preferred) or a couple of dashes of Tabasco sauce
1 large egg
12 ounces sharp cheddar cheese, grated or diced
Salt and pepper to taste
The topping:
1c panko (Japanese bread crumbs)
2 TB butter
Cook the paste al dente and drain.
In a 2 qt saucepan, melt the 3 TB butter and add the flour and mustard powder. Stir over medium heat for a few minutes until completely blended with no lumps. Do not let the flour brown. Stir in the milk, onion, paprika/hot sauce, and bay leaves. Simmer gently for 10 minutes. Add a healthy grinding of black pepper. Beat the egg and stir it in rapidly so that it does not set before mixed. Add 3/4 of the cheese and stir, over low heat, until it is all incorporated. Remove from heat; taste for salt and add some more if needed. Stir in the pasta and pour into a greased casserole dish. Distribute the remaining cheese on top.
Melt the 2 TB butter and stir in the panko. Sprinkle over the top of the casserole.
Bake at 350f for 45 minutes.
Note: I think of cavatappi as elbow macaroni that went to finishing school. They are a bit longer, and sort of spiral in shape, and the outside is ridged, better to hold sauces and the like. De Cecco is one brand that is widely available.