Preheat to 400˚F.
Butter a large hotel pan, smaller baking dishes, or individual gratins.
Melt the butter in a medium stockpot over medium heat and whisk in the flour. Continue to whisk until the mixture is fragrant, about 2 minutes. Whisk in the milk and the cream and continue to whisk until the mixture comes to a boil. Dip a wooden spoon into the sauce. It should coat the spoon and when a finger is run through the mixture, the line will remain.
Remove from the heat and let cool for 45 minutes. Stir in the hot sauce, mustard, and salt, followed by all of the cheese. Continue to stir as the cheese melts. (The cheese will not melt completely. There will still be some streaks.) Season the sauce with additional salt to taste, as needed.
Combine the macaroni with enough sauce to generously coat the noodles. You may have a bit left over. Stir in the pimentos until distributed.
Transfer to the baking vessel of choice and bake until warmed through in the center. Baking time will vary depending on the pan/dish it is baked in.
Recipe courtesy of Chefs Richard Park and Pamela Schafer.