Step 1: Build the base - Place the beef stew meat directly into the bottom of your slow cooker. Sprinkle evenly with salt and black pepper. Add the diced onion, carrots, and potatoes on top, spreading them out so everything cooks evenly.
Step 2: Mix the sauce - In a medium bowl, whisk together the tomato paste, flour, root beer, beef broth, Worcestershire sauce, soy sauce, brown sugar, dried thyme, and smoked paprika. Take a minute to make sure the tomato paste is fully blended so the sauce is smooth.
Step 3: Combine and cook - Pour the sauce mixture over the beef and vegetables in the slow cooker. Add the bay leaves, nestling them into the liquid. Cover with the lid and cook on low for 8 hours or high for 4 hours until the beef is tender and the potatoes are cooked through.
Step 4: Thicken the stew - About 20 minutes before serving, whisk the cornstarch and cold water together in a small bowl until smooth. Stir it into the stew, cover, and cook on high for another 15 to 20 minutes until the broth thickens slightly.
Step 5: Finish and serve - Remove the bay leaves and give the stew a good stir. Taste and adjust seasoning if needed. Ladle into bowls and sprinkle with fresh parsley before serving.