2Warm the milk in a medium saucepan over medium heat. melt the remaining 6 tablespoon butter in a high sided skillet over medium heat . When the butter bubbles, add the flour. cook, stirrng, 1 minute.
while whisking, slowly pour in the hot milk a little at a time to keep mixture smooth. Continue cooking, whisking constantly, until the mixture bubble and becomes thick, 8 to 12 minutes.
Remove the pan from the heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 3 cups cheddar cheese, and 1 1/2 cup gruyere, colby jack, 4 cheese. set the cheese sauce aside.
Cover a large pot of salted water, and bring to a boil. Cook the macaroni until the outside of the pasta is cooked and the inside is underdone, 2 to 3 minutes. Transfer the macaroni to a colander, rinse under cold running water, and drain well. Stir the macaroni into the reserved cheese sauce.
Pour the mixture into the prepared dish. Sprinkle the remaining 1 1/2 cup cheddar cheese, 1/2 cup gruyere and breadcrumbs over top . bake until golden brown, about 30 mintues.