Boil green beans in chicken broth for 10-20 minutes, then drain.
Melt butter and sauté onions and mushrooms for 1-2 minutes.
Mix mushroom soup, sautéed vegetables, green beans, garlic powder, black pepper, half of the onions, and half of the cheese. Transfer to a greased casserole dish.
Bake for 20 minutes, then top with remaining cheese and onions. Bake 5 more minutes until cheese melts.
Notes
For a softer texture, boil green beans longer; skip boiling if using canned green beans.