In a large skillet, heat one tablespoon of olive oil over medium-high heat. Add half of the beef cubes in a single layer, ensuring not to overcrowd the pan. Sear for about 45 seconds per side, until browned on all sides. This step is crucial for developing a deep, rich flavor in the stew. Transfer the seared beef and any juices to the slow cooker. Repeat with the remaining olive oil and beef.
In the same skillet, add the chopped onion and cook for 3-4 minutes, until softened. Add the minced garlic and cook for another 30 seconds until fragrant. Transfer the onion and garlic mixture to the slow cooker with the beef.
To the slow cooker, add the potatoes, carrots, celery, beef broth, tomato paste, Worcestershire sauce, salt, and pepper. Stir everything together until well combined. Gently place the bay leaves on top.
Cover the slow cooker and cook on low for 10 hours or until the beef is tender and easily falls apart. If you’re short on time, you can cook on high for 4-5 hours, but the low and slow method is recommended for the most tender results.
About 10-20 minutes before serving, if you prefer a thicker gravy, create a slurry by whisking together the cornstarch and cold water in a small bowl until smooth. Slowly stir the slurry into the stew. Cover and cook on high for another 10-15 minutes, or until the stew has thickened to your liking.