Heat the olive oil in a large skillet over medium-high heat. Add the beef chunks and brown them on all sides for about 5 minutes. Transfer the beef to the slow cooker.
Add the sliced onions and minced garlic to the same skillet. Cook for 3 minutes until the onions soften slightly, then scrape everything into the slow cooker.
Place the cubed potatoes, sliced carrots, thyme, and bay leaf into the slow cooker with the beef and onions.
Pour in the Guinness and beef broth. Stir the flour into 1/4 cup of the liquid until smooth, then add it back to the pot. Season with salt and pepper.
Cover and cook on low for 8 hours. Remove the bay leaf before serving.