This is the classic, a little rcher and a lot smoother. The extra-sharp cheddar and dash of mustard might remind you of Welsh rarebit.
1/2 pound (2 cups) dried elbow macaroni, cooked
5 tablespoons unsalted butter, divided
2-1/2 tablespoons all-purpose flour
2 cups whole milk
1 cup heavy cream
10 to 12 ounces coarsely grated, extra-sharp cheddar cheese
3/4 teaspoon salt
2 teaspoons Dijon-style mustard
scant 1/8 teaspoon cayenne pepper, if desired
1/3 cup Italian-style bread crumbs
1/3 cup finely grated Parmesan cheese
Preheat the oven to 350°F. Butter a 13x9-inch baking dish or 2-1/2-quart casserole. Melt 3 tablespoons of the butter over moderate heat in a large, preferably nonstick, saucepan. Add the flour and cook gently for 1 minute, stirring constantly. Increase the heat slightly, then add the milk, 1 cup at a time, whisking constantly as the sauce thickens. Whisk in the heavy cream. Stir in the cheddar cheese, about half at a time, and reduce the heat so that the sauce does not boil. Remove the pan from the heat and stir in the salt, mustard, and cayenne, if using, until smooth and evenly blended. Pour the sauce over the macaroni and stir well. Transfer the mixture to the prepared baking dish and smooth the top. Mix the bread crumbs and Parmesan cheese in a small bowl and then spread it evenly over the macaroni. Dot with the remaining 2 tablespoons of butter. Bake for 25 minutes, or until bubbly.