Season the beef with salt and pepper. Heat a skillet over medium-high heat and brown the beef in batches until all sides are seared. Transfer the beef to the slow cooker.
Add the mushrooms, onion, carrots, garlic, tomato paste, thyme, and bay leaves to the slow cooker. Pour in the Guinness and beef broth.
Stir everything together, cover, and cook on low for 7 to 8 hours until the beef is very tender.
About 30 minutes before serving, bring the water to a boil in a saucepan. Whisk in the polenta and a pinch of salt. Reduce heat and cook, stirring often, until thick and creamy, about 15 minutes.
Stir 3 tablespoons of butter into the polenta until melted. Remove the bay leaves from the stew and stir in the remaining tablespoon of butter.
Spoon the polenta into bowls, top with the stew, and sprinkle with chopped parsley.