Place the beef pieces in a bowl and toss them with the flour, salt, and pepper until evenly coated.
Add the coated beef to the slow cooker along with the carrots, parsnips, onion, potatoes, and garlic.
Pour the Guinness and beef broth over the meat and vegetables, then add the thyme and bay leaves.
Stir everything gently to combine and distribute the liquid.
Cover and cook on low for 8 hours or on high for 4 to 5 hours until the beef is fork-tender.
Remove the bay leaves, taste the broth, and adjust salt or pepper if needed.
Ladle the stew into bowls and sprinkle with the chopped parsley before serving.