Coat the beef: In a large bowl, combine flour, thyme, salt, pepper, and garlic powder. Add beef cubes and toss until coated. This will help thicken the stew later.
Layer the veggies: Place onions, carrots, celery, and potatoes in the bottom of the slow cooker.
Add beef and seasoning: Place coated beef on top of the vegetables. Add bay leaves.
Mix the broth: In a small bowl, whisk together beef broth, tomato paste, and Worcestershire sauce. Pour over the beef and vegetables.
Slow cook: Cover and cook on low for 8 hours or high for 4 hours, until beef is fork-tender and potatoes are cooked through.
Finish and serve: Remove bay leaves. Taste and adjust seasoning with additional salt and pepper if needed. Garnish with parsley before serving.