Saute onions in bacon grease then mix with the ketchup and brown sugar. Heat until bubbly. Crumble in the bacon.
Place green beans in a casserole dish or 9x13 pan and cover with sauce, do not stir. Bacon will remain on top. Place in oven uncovered at 250 degrees for 4 hours. It can be cooked in a hotter oven for a shorter time but is better when cooked slowly. Do not stir.