Green Bean Casserole From Scratch

fresh green beans, trimmed and cut in half*

butter, divided

sliced baby bella mushrooms

finely chopped yellow onion

garlic, minced

flour

low-sodium chicken or vegetable broth

half-n-half

salt

freshly ground black pepper

Greek yogurt

egg, lightly beaten

grated cheddar cheese

whole-wheat breadcrumbs (homemade** or store-bought)