Preheat the oven to 350°F. Have a 9 x 13-inch casserole dish ready.

In a deep-bottom saucepan, melt 7 tablespoons unsalted butter over medium heat.

Add 2 tablespoons dry mustard and 6 tablespoons all-purpose flour to the melted butter. Whisk briskly for about 2 minutes to cook the flour and form a roux.

Gradually pour in 3 cups whole milk while whisking continuously to prevent lumps. Continue whisking until the sauce thickens enough to coat the back of a spoon, about 2 to 3 minutes.

Remove the saucepan from the heat (or reduce heat to low) and stir in the cheeses: add 3 cups shredded Havarti and 3 cups shredded Gouda in handfuls, stirring until smooth after each addition. Stir in 1/4 teaspoon kosher salt and 1/4 teaspoon white ground pepper.

Meanwhile, cook 1 pound elbow macaroni (or any short pasta) in boiling water until just al dente according to package directions. Drain the pasta thoroughly.

Pour the drained pasta into the pot with the cheese sauce and stir until the pasta is evenly coated.

Transfer the mac and cheese mixture to the 9 x 13 casserole dish and spread it evenly. Sprinkle 2 tablespoons breadcrumbs evenly over the top. Cover the dish with foil.

Bake covered for 15 minutes. Remove the foil and return the dish to the oven; bake an additional 10 to 15 minutes, or until the top is bubbling and golden brown.

Remove from the oven and let rest a few minutes. If desired, sprinkle 2 tablespoons chopped parsley on top before serving.