3 tablespoons unsalted butter, softened
10 slices white sandwich bread
12 ounces cheddar cheese, shredded (3 cups)
8 large eggs
3 cups whole milk
1 small onion, grated
1 teaspoon salt
1/2 teaspoon pepper
1 teaspoon dry mustard
1/2 teaspoon hot sauce
Grease a 13-by-9-inch baking dish. Spread butter evenly over 1 side of bread slices, then cut into 1-inch pieces. Scatter half of bread evenly in prepared dish and sprinkle with half of cheddar. Repeat with remaining bread and cheese.
Whisk eggs, milk, onion, salt, pepper, mustard and hot sauce together in bowl until well combined. Pour egg mixture evenly over bread and press lightly on bread to submerge. Wrap dish tightly with plastic wrap and refrigerate at least 8 hours.
Adjust oven rack to middle position and heat oven to 350 degrees. Unwrap casserole and bake until puffed and golden, about 1 hour. Serve immediately. Serves 6 to 8.