Butter a 9-inch square or other shallow 1 1/2-quart casserole dish. Arrange bread in an even layer, cutting some slices in half if necessary so that they all fit.
With a fork or whisk, beat together eggs, milk, minced onion, Worcestershire sauce, salt and pepper. Stir in shredded Cheddar cheese, then pour mixture evenly over bread.
Preheat oven to 375°. Meanwhile, set dish aside and let egg mixture set over bread at room temperature for 15 minutes.
Bake for about 30 minutes or until top is puffed and lightly browned. Sprinkle additional shredded Cheddar cheese on top if desired. Serve hot.