Turn the slow cooker on to low to pre-warm while gathering and preparing the ingredients. Cut the beef and potatoes in 2cm dice. Brown the beef and onion well in a dash of oil in a hot frying pan. This will be best done in 2-3 batches. Into the pre-warmed slow cooker, put the beef, potatoes, onion, tomatoes, olives and garlic. In a jug stir together the canned tomatoes, stock or water, sherry and paprika. Pour into the crock pot, stir to mix and cover with the lid. Cook on low for 6 hours or high for 3½-4 hours without lifting the lid. Once the cooking time is finished, check the beef for tenderness and continue to cook if required. Season well with salt and pepper. Season the stew scattered with a little parsley and extra olives if wished.