Cook ground beef and onion in a skillet until beef is browned and onions are tender. Drain. Add tomato soup, green beans, and pepper to the ground beef, blend well. Place mixture in a greased 2 qt. casserole dish.
In a sauce pan combine potatoes, 1 t. salt, and enough water to cover. Heat to boiling. Cover and reduce heat, simmer for 30 minutes or until potatoes are fork tender. Drain.
In a large bowl combine cooked potatoes, 1 t. salt, margarine, and pepper. Beat on low speed until fluffy. Add milk and continue beating on medium speed until mixture is moist (add extra milk if needed). Beat 2 more minutes or until smooth.
Spread potatoes over beef mixture and bake uncovered at 350 degrees for 45 minutes.