Bring 2 gallons of water with 1/4 cup salt to a boil in a large pot. Cook the pasta to al dente, about 8 minutes. Drain and cool immediately in the refrigerator or on a flat cold baking sheet.
Melt 4 ounces of the butter in a separate pot. Whisk in the flour, stirring constantly for about 5 minutes, making sure there are no lumps. Stir in the milk, onions, paprika, mustard and bay leaves and simmer for 10 minutes. Remove the bay leaves and stir in the cheeses in batches, being careful not to break the sauce. Season with the salt and pepper.
Fold the macaroni into the hot cheese and pour into a 9-inch cast-iron casserole dish. Top with a big pinch of Cheddar. Melt the remaining butter in a pan and stir in the breadcrumbs, herbs and some salt and pepper. Scatter the topping over the cheesy mac and bake until lightly brown on top, 12 to 15 minutes. Serve.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.