Heat vegetable oil in a large pot over medium-high heat.
Season the beef chunks with salt and pepper, then add them to the pot in batches.
Once browned, remove the beef and set aside. In the same pot, add chopped onions, carrots, and celery.
Stir in minced garlic and tomato paste, cooking for an additional minute.
Deglaze the pot with the Guinness, scraping up any brown bits.
Return the beef to the pot, then add the beef broth, thyme, bay leaves, and bring to a boil.
Reduce the heat to low, cover, and let it simmer for about 2 to 2.5 hours until the beef is tender.
During the last 30 minutes of cooking, taste and adjust seasoning more salt and pepper, if necessary.
Remove and discard the bay leaves and thyme sprigs before serving the stew hot.