Prep the Beef: In a medium bowl, toss the beef cubes with the flour, salt, and pepper until evenly coated.
Sear the Beef: Heat the olive oil in a large skillet over medium-high heat. Add half of the beef in a single layer and sear for 2-3 minutes per side, until a deep brown crust forms. Transfer the seared beef to your 6-quart or larger slow cooker. Repeat with the remaining beef.
Sauté Aromatics: Reduce the skillet heat to medium. Add the chopped onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for 1 more minute until fragrant. Transfer the onion and garlic to the slow cooker.
Load the Crockpot: Add the carrots, celery, and potatoes to the slow cooker with the beef and onions.
Make the Gravy: In a bowl, whisk together the beef broth, tomato paste, Worcestershire sauce, and dried thyme. Pour this mixture into the slow cooker. Tuck the bay leaf into the stew.
Slow Cook: Stir everything gently to combine. Cover and cook on LOW for 8-10 hours or on HIGH for 4-5 hours, until the beef and vegetables are very tender.
Finish & Serve: Remove and discard the bay leaf. Stir in the frozen peas (they will cook in the residual heat). Taste and season with additional salt and pepper if needed. Serve hot, garnished with fresh parsley.