Cook and stir ground beef in a skillet over medium-high heat until beef is browned, about 10 minutes. Drain fat. Mix in green beans, cream of mushroom soup, and onion.
Pour beef mixture into a 2 quart casserole dish and top with Cheddar cheese. Spread mashed potato on top.
Bake in the preheated oven until the pie is hot, and the mashed potatoes are golden brown, about 30 minutes.
