6 tablespoons unsalted butter, plus more for dish
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons sea salt
1/2 teaspoon nutmeg
1/2 teaspoon black pepper
1/2 teaspoon cayenne pepper
4 1/2 cups (about 18 ounces) shredded sharp white cheddar
2 cups (about 8 ounces) shredded Gruyere
1 pound rice macaroni
1. Heat the oven to 375 degrees. Butter a 3-quart casserole dish; set aside. In a medium saucepan set over medium heat, heat milk. Melt remaining 6 tablespoons butter in a high-sided skillet over medium heat. When butter bubbles, add flour. Cook, stirring, 1 minute.
2. Slowly pour hot milk into flour-butter mixture while whisking. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick.
3. Remove the pan from the heat. Stir in salt, nutmeg, black pepper, cayenne pepper, and all cheeses. Set cheese sauce aside.
4. Fill a large saucepan with water. Bring to a boil. Add macaroni; cook 2 to 3 fewer minutes than manufacturer's directions, until outside of pasta is cooked and inside is underdone. Transfer the macaroni to a colander, rinse under cold running water, and drain well. Stir macaroni into the reserved cheese sauce.
5. Pour the mixture into the prepared casserole dish. Bake until browned on top, about 35 minutes. Transfer dish to a wire rack to cool for 5 minutes; serve.