Cut the lamb into 1 inch cubes and chop the onions. Cook in the butter and oil until the onions are soft and golden and the lamb is lightly browned. Stir in the flour and cook for 2 minutes. Add seasoning, marjoram, stock and tomatoes with their juice. Bring to the boil, stirring well. Pour into crockpot, cover and cook on High for 30 minutes, then on Low for 6-7 hours.