Prepare the night before.
Butter a 9" x 13" or bigger pyrex dish.
Cook sausage in a medium fry pan until thoroughly cooked- break into crumbles with spatula while cooking. Drain.
Saute mushrooms. Drain.
Butter bread- leave on crust. Cut into 1" squares and line dish buttered side down. Press well to fit in dish.
Beat eggs, milk, salt, and pepper. Add cheddar and jack cheeses and stir. Gently pour over bread. Top with mushrooms, then sausage.
Cover and refrigerate overnight.
Next morning, take out of refrigerator and remove cover.
Preheat oven to 350º.
Bake at 350º for 1 hour.