Toss the beef cubes with the flour, salt, and pepper until evenly coated.
Heat a skillet over medium heat and brown the beef in batches until all sides are seared.
Add the onion and garlic to the skillet and cook for two minutes until softened.
Transfer the beef mixture to the slow cooker.
Add the carrots, parsnips, potatoes, stout, beef broth, tomato paste, thyme sprigs, and bay leaves.
Stir gently to combine all ingredients.
Cover and cook on low for 8 hours until the beef is tender and the gravy has thickened.
Remove the bay leaves and thyme stems before serving.