Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Grate all cheeses and set aside (reserve about 1/2 cup of the mixed cheese blend for the top layer).
- Boil the macaroni according to package directions until perfectly al dente (slightly undercooked). Drain immediately and set aside.
- Make the Roux: Melt the 4 Tbsp of butter in a medium saucepan over medium heat. Whisk in the flour continuously for 2 minutes to form a smooth paste (roux). Do not allow it to brown heavily.
- Create the Béchamel: Gradually whisk in the warm milk, adding a little at a time to prevent lumps. Increase heat slightly and continue whisking until the sauce thickens enough to coat the back of a spoon (about 5–8 minutes).
- Season and Melt: Remove the sauce from the heat. Whisk in the mustard, nutmeg, salt, and pepper. Add the main portion of the grated cheese blend slowly, handful by handful, stirring gently until completely smooth. Do not reheat the sauce once the cheese is incorporated.
- Combine: Gently fold the drained pasta into the cheese sauce until evenly coated. Pour the mixture into the prepared baking dish.
- Prepare Topping: In a small bowl, toss the Panko breadcrumbs with the 2 Tbsp melted butter and the optional Parmesan. Sprinkle the reserved cheese blend over the pasta, then evenly top with the buttered Panko.
- Bake for 30–35 minutes, or until the sauce is bubbling vigorously and the Panko topping is golden brown and crisp.
- Rest the mac and cheese for 10 minutes before serving to allow the sauce to set slightly.