Heat the vegetable oil in a large pot over medium-high heat.
Season the beef cubes with salt and pepper, then add them to the pot.
Sear the beef for about 5-7 minutes, turning occasionally, until browned on all sides.
Remove the browned beef from the pot and set it aside.
In the same pot, add the chopped onion, carrots, and celery.
Add the minced garlic and tomato paste, cooking for another 2 minutes.
Pour in the Worcestershire sauce, beef broth, and Guinness stout, scraping the bottom of the pot to deglaze.
Return the beef to the pot along with thyme and bay leaves.
Bring the mixture to a boil, then reduce heat to low and cover.
Simmer for at least 2 to 2.5 hours until the beef is fork-tender.
Taste and adjust seasoning with salt and pepper as necessary before serving.
Serve hot, garnished with fresh parsley.