Pat the beef cubes dry with paper towels. Season evenly with kosher salt and freshly ground black pepper. Place the all-purpose flour on a large plate and dredge the beef cubes, shaking off excess flour. Set the dredged beef aside on a tray.

Braising Aromatics & Vegetables

Brown the Beef

In a large Dutch oven or heavy-bottomed pot over medium-high heat, add 1 tablespoon vegetable oil and 1 tablespoon unsalted butter. When the fat is shimmering, add half of the floured beef in a single layer (do not overcrowd). Brown on all sides, about 3–4 minutes per batch. Transfer browned beef to a plate and repeat with remaining oil, butter and beef.

Sauté Vegetables

Reduce heat to medium. Add the diced onion, carrots and celery to the pot. Cook, stirring occasionally, until the onion is translucent and edges start to color, about 6–8 minutes. Add minced garlic and cook 30–45 seconds until fragrant. If using mushrooms, add them now and cook until they release moisture and brown, about 4–5 minutes. Stir in the tomato paste and cook 1–2 minutes to remove raw taste.

Liquids, Stock & Seasoning

Deglaze and Build Braise

Pour the Guinness (12 oz) into the pot to deglaze, scraping up browned bits from the bottom with a wooden spoon. Let the beer simmer 2 minutes to reduce slightly. Add the beef stock and Worcestershire sauce, then return the browned beef and any accumulated juices to the pot. Add bay leaves, thyme sprigs and optional brown sugar. Bring to a gentle simmer.

Braise

Cover the pot with a tight-fitting lid and transfer to a preheated oven at 325°F (160°C), or keep on the stovetop at a low simmer. Braise until the beef is very tender and easily shredded with a fork, about 2 to 2½ hours. Check once at about 1¼ hours to ensure liquid level is adequate; add up to ¼ cup water or stock if it looks dry.

Finishing & Thickening

Thicken and Finish

When beef is tender, remove the pot from the oven and place on the stovetop. Transfer bay leaves and thyme sprigs to discard. Skim off excess fat from the surface if desired. In a small bowl, whisk cornstarch with cold water to make a slurry. Bring the braise to a simmer over medium heat and stir in the slurry, simmering for 2–3 minutes until sauce thickens slightly. Stir in the vinegar and taste; adjust seasoning with additional salt, pepper or a pinch more brown sugar if needed.

Rest and Serve

Let the stew rest 10 minutes off heat to allow flavors to settle. Sprinkle with chopped fresh parsley before serving. Serve the braised Guinness beef over mashed potatoes, buttered noodles, or with crusty bread.