Spicy Mac and Cheese

Name: Spicy Mac and Cheese
Ingredients: 1 tablespoon unsalted butter; 4 slices white bread, cut into 1/2-inch squares; Kosher salt; 8 ounces Cheddar cheese, shredded; 8 ounces Colby cheese, shredded; 8 ounces pepper Jack cheese, shredded; 2 cups elbow pasta, cooked until almost al dente; 2 teaspoons all-purpose flour; 1/2 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 1/2 teaspoon cayenne pepper; 1/2 teaspoon dry mustard powder; 1/8 teaspoon grated nutmeg; 1/4 cup sour cream; 1 egg, beaten; 1/4 cup grated Vidalia or sweet onion; 1 cup heavy cream; 1 cup half-and-half