Toss beef cubes with flour until coated.
Add beef, onion, carrots, and celery to slow cooker.
Whisk together beef broth, tomato paste, Worcestershire sauce, garlic, thyme, salt, and pepper, then pour over beef and vegetables.
Cover and cook on low for 8 hours until beef is fork-tender.
At the 7-hour mark, mix flour, baking powder, Italian seasoning, and salt in a bowl.
Stir in melted butter and milk until a soft, sticky dough forms.
Drop large spoonfuls of dough on top of stew to make 8-10 dumplings.
Cover, switch to high heat, and cook 50 minutes to 1 hour until dumplings are puffed and cooked through.
Test dumplings with a toothpick for doneness.
Serve immediately garnished with parsley if desired.