Season the beef cubes with salt and pepper, then toss them with the flour until evenly coated.
Heat a large skillet over medium-high heat and brown the beef in batches for 3 to 4 minutes per side. Transfer the browned beef to the slow cooker.
Add the celery root, onion, carrots, and garlic to the slow cooker.
Pour in the Guinness and beef broth. Stir in the thyme and bay leaves.
Cover and cook on low for 7 to 8 hours until the beef is fork-tender.
Remove the bay leaves. Stir in the whole-grain mustard until fully combined.
Taste and adjust salt if needed. Sprinkle with chopped parsley before serving.