Pat the beef cubes dry with paper towels. In a shallow dish, combine 1/4 cup flour, 1/2 tsp salt, and 1/4 tsp black pepper. Dredge the beef cubes in the flour mixture, shaking off any excess.

Heat olive oil in a large skillet over medium-high heat. Sear the beef in batches until browned on all sides, about 2-3 minutes per batch. Do not overcrowd the skillet. Transfer browned beef to the slow cooker.

Add the chopped onion to the same skillet (add a little more oil if needed) and cook until softened, about 3-5 minutes. Stir in the minced garlic and cook for another minute until fragrant.

Pour in the red wine (if using) to deglaze the pan, scraping up any browned bits from the bottom. Let it simmer for 1-2 minutes until slightly reduced. If not using wine, skip this step and proceed.

Stir in the tomato paste and cook for 1 minute.

Add the carrots, celery, and potatoes to the slow cooker with the beef.

Pour the onion-garlic mixture (and wine reduction if applicable) from the skillet into the slow cooker. Add beef broth, Worcestershire sauce, dried thyme, remaining 1/2 tsp salt, remaining 1/4 tsp black pepper, and bay leaves.

Stir everything gently to combine. Ensure all ingredients are submerged as much as possible.

Cover the slow cooker and cook on LOW for 7-8 hours, or on HIGH for 3.5-4 hours, until the beef is fork-tender and vegetables are soft.

If you prefer a thicker stew, whisk together cornstarch and cold water in a small bowl until smooth to create a slurry. Stir the slurry into the hot stew during the last 30 minutes of cooking (on high heat if possible), or remove the bay leaves, transfer a cup of liquid to a saucepan, bring to a simmer, whisk in the slurry, cook for 1-2 minutes until thickened, then return to the slow cooker and stir.

Remove bay leaves before serving. Ladle the hearty beef stew into bowls, garnish with fresh chopped parsley, and serve hot.