Sear the beef. Generously season the beef with salt and pepper. Heat 1 tablespoon oil in a large stockpot over high heat. Add half of the beef to pan and sear, flipping the beef once it has developed a good brown sear on the bottom then repeating on multiple sides. Transfer the beef to a clean plate. Add an additional 1 tablespoon oil to the pot, add the remaining beef, sear, and transfer to a clean plate.
Sauté the onion and garlic. Reduce heat to medium-high and add the remaining tablespoon of oil to the stockpot. Add onion and sauté for 5 minutes, stirring occasionally. Add garlic and sauté for 1 minute, stirring occasionally. Stir in the flour and cook for 1 minute, stirring frequently.
Simmer. Pour in the Guinness and use a wooden spoon to scrape the bottom of the pan to lift up those yummy brown bits. Stir in the beef stock, carrots, potatoes, tomato paste, bay leaf, Worcestershire, dried thyme, and the cooked beef (along with any of its accumulated juices). Cover the pot and simmer on low for 1 1/2 hours, stirring occasionally, until the beef is completely tender and the potatoes are soft.
Season. Discard the bay leaf. Taste season the stew with additional salt and pepper as needed, plus extra Worcestershire or dried thyme if desired.
Serve. Serve warm, garnished with a sprinkle of fresh parsley if desired, and enjoy!