1. Heat oven to 400 degrees F.
2. On stove top, bring 4 quarts of water and ham to boiling in a pot. Once water begins to boil, remove the ham to a plate. Add the onion and macaroni to the pot. Return the water to boiling; cook for 8 minutes, stirring the macaroni occasionally.
3. Coarsely chop the ham, discarding any fat and bones.
4. Drain the macaroni and onion; return macaroni and onion to the pot. Add the ham, Cheddar cheese, sour cream and cream of chicken soup. Pour into 13 x 9 x 2-inch baking dish. Pour melted margarine over top. Sprinkle with bread crumbs.
5. Bake in 400 degrees F oven 30 minutes or until heated through and top is golden brown. Let stand 5 minutes before serving.