Preheat oven to 350 degrees. Melt butter in a large skillet over medium heat. Add onions and mushrooms and cook, stirring occasionally, until just tender. Add garlic and cook another 1-2 minutes, then remove from heat and set aside.

Place green beans and chicken broth in a medium saucepan and bring to a boil. Cook about 8 minutes until beans are crisp-tender and drain.

Spray a 2-quart baking dish with cooking spray. Place the soup, mushroom mixture, green beans, Parmesan, salt, and pepper in a large bowl and stir to combine. Transfer mixture to the prepared baking dish and top with crispy onion strings. Bake at 350 degrees for 20 minutes, until casserole is heated through.