Add the cubed beef, flour, salt and pepper to a large ziplock bag. Toss to coat. Heat oil in a large skillet over high heat. Brown the beef in two batches until caramelized.
Add the beef to the slow cooker along with one onion, 3 celery stalks, 3 carrots, garlic, tomato paste, 2 cups broth, 1 pint of Guinness, Worcestershire sauce, potatoes, bay leaves, and Italian seasoning.
Allow the beef stew to cook in the Crockpot for eight hours on low. Season with salt and pepper.