Pat the beef cubes dry with paper towels. In a large bowl or on a plate, toss the beef with flour, 1 teaspoon salt, and 1/2 teaspoon black pepper until evenly coated.

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

Reduce heat to medium. Add chopped onion to the same skillet and cook until softened, about 5 minutes, scraping up any browned bits from the bottom of the pan. Add minced garlic and cook for another minute until fragrant.

Pour in beef broth, tomato paste, and Worcestershire sauce. Stir well, scraping the bottom of the skillet to loosen any remaining flavorful browned bits. Bring to a simmer, then stir in dried thyme and bay leaves.

Add the potatoes, carrots, and celery to the slow cooker with the browned beef. Pour the hot broth mixture from the skillet over everything in the slow cooker. Ensure the ingredients are mostly submerged; add a little more broth or water if necessary.

Cover and cook on LOW for 7-8 hours, or on HIGH for 3-4 hours, until the beef is fork-tender and the vegetables are soft.

If you prefer a thicker stew, in a small bowl, whisk together the cornstarch and cold water until smooth. Stir the cornstarch slurry into the stew during the last 30 minutes of cooking (if on HIGH) or during the last hour (if on LOW). Cook until thickened.

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