01 - Preheat oven to 325°F. Pat beef roast thoroughly dry with paper towels, then season generously on all sides with kosher salt and black pepper.
02 - Heat vegetable oil in a large Dutch oven over medium-high heat until shimmering. Sear the seasoned roast on all sides until deeply browned, approximately 3-4 minutes per side. Transfer browned beef to a plate and set aside.
03 - In the same pot, add quartered onions, carrots, celery chunks, and minced garlic. Cook, stirring frequently, for 3-4 minutes until vegetables begin to soften and release their aromas.
04 - Stir in tomato paste and cook for 1 minute, allowing it to darken slightly and develop a rich, caramelized flavor base.
05 - Return the seared beef to the pot. Pour in beef broth and Guinness if using. Add Worcestershire sauce, dried thyme, and bay leaves. Bring the mixture to a gentle simmer, scraping up any browned bits from the bottom of the pot.
06 - Cover the Dutch oven tightly with its lid. Transfer to the preheated oven and roast for 2 hours, allowing the beef to begin breaking down and developing tenderness.
07 - Remove pot from oven briefly. Add halved baby potatoes, submerging them in the cooking liquid. Cover and return to oven for an additional hour, until beef is fork-tender and potatoes are easily pierced with a knife.
08 - Discard bay leaves. Slice or shred the tender beef against the grain. Serve hot, arranged with vegetables and generously ladled with the rich pan juices. Garnish with freshly chopped parsley.