Butter the bread the place 8 slices, buttered side down, in baking dish.
Sprinkle with layer of grated cheese.
Place the remaining 8 slices, buttered side up, on top and add another layer of grated cheese.
Mix together the eggs, milk, salt and mustard mixture; pour over bread slices.
It’s best to make this ahead of time and let it set, refrigerated, overnight.
Bake at 350°F for about 1 hour or until set.