Preheat oven to 350 degrees. Prepare two regular 9x13 casserole dishes or one 11x18 casserole dish with nonstick cooking spray.
In a large skillet over medium-high heat, stir together the cooked ground beef, mushrooms, and cream cheese. (The cream cheese melts easier if already at room temperature.) Add the beef broth, heavy cream, garlic powder, salt, and black pepper. Stir to combine.
Meanwhile, place green beans in a large, microwave-safe bowl with ¼ cup water. Cook on high for 5 to 8 minutes or until the green beans are bright green and crisp-tender. Drain.
Stir together the green beans and the beef mixture.
Divide evenly between the two casserole dishes or place in one large casserole dish.
Top with cheese.
Bake for 25 to 30 minutes or until the beans are tender and the cheese has melted.
Allow the casserole to rest 5 minutes before serving.