Brown onion and hamburger together. If you already have hamburger cooked and frozen, cook onion in small amount of oil till tender.
Add black beans, tomatoes and chili powder. Heat over medium heat till hot.
Pour into greased 2 quart casserole dish.
Top with cheese.
Top with mashed potatoes.
Dot potatoes with butter.
Bake 400 degrees for 15-20 minutes.