Preheat the oven to 350 degrees. In a large skillet, melt butter over medium heat.

Saute onions and mushrooms until just tender, stirring occasionally. Add garlic and cook for another 1-2 minutes, then remove from heat and set aside.

In a medium saucepan, combine green beans with chicken broth. Bring to a boil.

Continue cooking for about 8 minutes until beans are crisp-tender and drain.

Prepare a 2-quart baking dish by spraying it with cooking spray. In a large bowl, combine the soup, mushroom mixture, green beans, Parmesan, salt, and pepper.

Transfer the mixture to the prepared baking dish and top with crispy onion strings.

Bake at 350 degrees for 20 minutes, or until the casserole is heated through.