Heat the olive oil in a non stick pan. While that is heating, place the flour in a large bowl add season with salt and pepper to taste, mix in the beef to coat. When the oil is hot, add the beef to the pan in batches to brown all sides, not cooking it through just browning it a bit. Place browned beef in your slow cooker.
Once all the beef is browned and in the slow cooker, pour in the stock, diced tomatoes (these can be canned or fresh), Worcestershire sauce, vinegar, and stir, then add the potatoes, carrots, onion, celery, oregano, garlic, and bay leaf.
Stir to mix then cover and set the slow cooker to Low setting and cook for 8 hours.
Whisk together the cornstarch and water, pour into the slow cooker and stir to mix into the stew, cover the pot and set to High setting for another 30 minutes to an hour to thicken the stew.
Serve with biscuits or bread.