Pat the beef cubes dry with a paper towel. In a large bowl, whisk together the flour, salt, and pepper. Add the beef and toss until evenly coated.

Heat olive oil in a large skillet or Dutch oven over medium-high heat. Working in batches to avoid overcrowding, sear the beef on all sides until a deep brown crust forms, about 5-7 minutes per batch. Transfer the seared beef to the crockpot.

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

Pour the onion and garlic mixture into the crockpot. Add the beef broth, Worcestershire sauce, thyme, rosemary, and bay leaves. Stir everything together. Then, add the potatoes, carrots, and celery to the crockpot and stir to combine.

Cover the crockpot and cook on LOW for 8-10 hours or on HIGH for 4-5 hours, until the beef is fall-apart tender.

About 30 minutes before serving, if you desire a thicker gravy, whisk together the cornstarch and cold water in a small bowl to create a slurry. Stir the slurry into the stew, turn the crockpot to HIGH, and cook, covered, for 20-30 minutes or until the gravy has thickened.

Remove the bay leaves before serving. Taste and adjust seasoning with more salt and pepper if needed. Garnish with fresh parsley and serve hot.