The Best Green Bean Casserole

slices white sandwich bread (torn into quarters)

butter (softened)

Salt and freshly ground black pepper

canned fried onions ((about 6 ounces))

Salt and freshly ground black pepper

green beans (ends trimmed, halved)

butter

white button mushrooms (wiped clean and sliced or broken into pieces)

garlic (minced)

all-purpose flour

chicken broth

heavy cream