Make the bechamel sauce, recipe follows, in a large 12-inch oven-safe skillet.
Bring a pot of salted water to a boil and cook the pasta until nearing al dente, but not fully cooked; drain. Preheat the oven to 400 degrees F.
Over low heat, stir the dry mustard and cayenne pepper into the bechamel sauce. Gradually add the Cheddar, Gruyere, and 1 cup Parmesan cheese, stirring constantly until all of the cheese has melted into the bechamel sauce. Add an additional cup of milk and cream. Taste and adjust the seasonings, if needed.
In another large skillet, cook the bacon until crispy and golden brown. Remove the cooked bacon from the skillet, reserving 3 tablespoons rendered bacon fat.
Add the cooked pasta to the bechamel and stir to coat evenly with the sauce. In a small bowl, combine the bread crumbs, chopped parsley, and reserved bacon fat. Coat the bread crumbs and chopped parsley with bacon fat and sprinkle the mixture over the top of the macaroni. Sprinkle the top with 1/2 cup Parmesan cheese.
Bake until the top is browned and the sauce is bubbling, 20 to 25 minutes. Top with crumbled cooked bacon. Let sit 5 minutes before serving.
To make the Bechamel:
Melt the butter in a heavy-bottomed saucepan or an oven-safe skillet.
Whisk in the flour and cook, stirring constantly, until the paste cooks and bubbles a bit, avoid browning, about 2 minutes. Add the hot milk, continuing to stir as the sauce thickens. Bring the sauce to a boil. Add the nutmeg, salt, and pepper, to taste. Reduce the heat, and cook, stirring for 2 to 3 minutes more. Remove from the heat.