Season the beef cubes with salt and pepper, then toss them with the flour until lightly coated.
Heat a skillet over medium heat and brown the beef in batches until all sides are seared. Transfer the beef to the slow cooker.
Add the onion, garlic, turnips, carrots, and potatoes to the slow cooker.
Stir in the tomato paste, thyme, and beef broth until everything is combined.
Pour the stout beer over the mixture and stir gently.
Cover and cook on low for 7 to 8 hours until the beef and vegetables are fork-tender.
Stir in the frozen peas during the final 15 minutes of cooking.
Taste and adjust salt and pepper if needed, then sprinkle with fresh parsley before serving.