Pat beef cubes dry with paper towels and season all over with salt and pepper. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Working in two batches to avoid crowding, sear beef on all sides until deeply browned, about 3-4 minutes per batch. Transfer seared beef to the slow cooker.
Reduce skillet heat to medium, add remaining 1 tablespoon oil, and sauté onion until softened, about 4 minutes. Stir in garlic and tomato paste and cook 1 minute more. Pour in a splash of beef broth to deglaze the pan, scraping up browned bits. Transfer everything to the slow cooker.
To the slow cooker, add remaining beef broth, red wine, Worcestershire sauce, thyme, and bay leaves. Nestle the potatoes, carrots, and celery around the beef. Stir gently to distribute.
Cover and cook on LOW for 8 hours or on HIGH for 4 hours, until beef is fork-tender, vegetables are cooked through, and beef reaches an internal temperature of at least 145°F (63°C).
About 15 minutes before serving, remove and discard bay leaves. Stir the cornstarch slurry into the stew, turn the slow cooker to HIGH if it was on LOW, and cook uncovered for 10-15 minutes until thickened. Taste and adjust salt and pepper.
Ladle stew into bowls, sprinkle with fresh parsley, and serve hot.