Martha's Macaroni-and-Cheese

Ingredients

• 8 tablespoons (1 stick) unsalted butter, plus more for casserole
• 6 slices white bread, crusts removed, torn into 1/4- to l/2-inch pieces
• 5 1/2 cups milk
• 1/2 cup all-purpose flour
• 2 teaspoons coarse salt, plus more for water
• 1/4 teaspoon ground nutmeg
• 1/4 teaspoon freshly ground black pepper
• 1/4 teaspoon cayenne pepper
• 4 1/2 cups (about 18 ounces) grated sharp white cheddar cheese
• 2 cups (about 8 ounces) grated gruyère or 1 1/4 cups (about 5 ounces) grated pecorino romano cheese
• 1 pound elbow macaroni