Heat the olive oil in a large skillet over medium-high heat. Add the beef pieces and brown them on all sides for about 5 minutes.
Sprinkle the flour over the beef and stir to coat evenly, cooking for 1 minute more.
Transfer the beef to the slow cooker. Add the onion, garlic, potatoes, carrots, and parsnips.
In a bowl, whisk together the Guinness, beef broth, tomato paste, thyme, bay leaves, salt, and pepper until smooth.
Pour the liquid mixture over the ingredients in the slow cooker and stir to combine.
Cover and cook on low for 8 hours or on high for 4 to 5 hours, until the beef and vegetables are fork-tender.
Remove the bay leaves before serving.