Butter a 6 cup deep, straight sided baking dish.
Layer ⅓ of each bread, meat, and cheese in the dish.
Repeat to make 2 layers.
Beat eggs with the milk, onion, soup base and mustard until blended.
Pour egg mixture over strata.
Cover and refrigerate for at least 3 hours or overnight.
Bake uncovered at 350° F oven for 1 & ½ hours or until puffy and golden. Serves 6 – 8 people.