3tablespoonsunsalted buttersoftened (plus extra for baking dish)
10sliceshigh-quality white sandwich bread
12ouncescheddar cheeseshredded (about 3 cups)
1small oniongrated over large holes of box grater
Butter the bottom and sides of a 9x13 baking dish (or similar size casserole dish). Spread the 3 tablespoons of butter evenly over one side of each slice of bread. Cut the bread into 1-inch pieces. Scatter half of the bread pieces evenly in the prepared dish and sprinkle with half of the cheese. Repeat with remaining bread and cheese.
In a large bowl, whisk the eggs until combined, then whisk in the milk, onion, salt, pepper, mustard, and hot sauce. Pour the mixture over the bread. Press lightly on bread to submerge.
Cover the dish tightly with plastic wrap and refrigerate for at least 8 hours or up to 24 hours.
Preheat oven to 350°F/175°C. Unwrap casserole and bake until puffed and golden, about 1 hour.
Pepperidge Farm Hearty White bread is recommended for this recipe.