Preheat your oven to 350°F (175°C)
Steam the green beans until they're crisp-tender, about 4-5 minutes. Drain and set aside.
In a skillet over medium heat, sauté the mushrooms, onions, and garlic until tender, approximately 5 minutes.
In a separate bowl, combine the cream of mushroom soup, milk, chicken broth, salt, and pepper.
Add the sautéed mushroom mixture and green beans to the soup mixture and stir well.
Transfer the mixture to a baking dish coated with cooking spray.
In a small bowl, mix the breadcrumbs and Parmesan cheese. Sprinkle this mixture evenly over the casserole.
Bake for 25-30 minutes, or until the top is golden and the casserole is bubbly.