The Best Creamy Mac & Cheese

unsalted butter, plus more for casserole dish

slices white bread, crusts removed, chopped into 1/4- to l/2-inch pieces

and 3/4 cups milk

all-purpose flour

coarse salt, plus more for water

ground nutmeg

freshly ground black pepper

cayenne pepper

and 1/4 cups (about 9 ounces) sharp white cheddar cheese, grated

(about 4 ounces) grated Gruyère or 2/3 cups grated Pecorino Romano cheese

elbow macaroni or other spiral noodle