Pull backs off live crabs. Rinse out guts with garden hose. Cut off dead men fingers, apron and mouth. Chop off crab legs and discard, but save claws. Scrub crabs with small brush to remove all dirt and rinse well. Place crabs in large pot and add about 2 cups of water. Add about 1 tablespoon of fat meat grease or bacon grease. Sprinkle crabs with salt, pepper and a dash of Old Bay. Add onion and potatoes. Cover and cook over medium-high heat, adding water if necessary, for about 40 minutes. Make dumplings and place over top of crabs. Cover and cook over low heat 15 minutes. Remove dumplings, crabs, potatoes and make gravy with flour and water paste.