Butter bread on a wooden board, cut into small cubes. Alternate layers of bread cubes and Cheddar cheese in a buttered 8-cup casserole. Blend eggs, milk, dry mustard, salt and pepper in a medium-size bowl; pour over bread-cheese mixture. Cover with plastic wrap; refrigerate overnight. Bake in moderate oven (350°) 1 hour, or until puffed and golden Garnish with watercress, bacon curls and crushed potato chips, if you wish.