Sear the Beef: Heat olive oil in a large Dutch oven over medium-high heat. Sear the beef chunks in batches until deeply browned on all sides. Remove and set aside. (I also always soak the leeks then dry them off before using)
Sauté the Veggies: Add butter and leeks to the pot. Season lightly with salt and cook until softened and fragrant.
Build the Base: Stir in flour, bouillon, and tomato paste. Pour in the Guinness and stir to deglaze the pan, scraping up all those flavorful bits.
Add the Rest: Stir in the seasonings (salt, pepper, onion powder, garlic powder, thyme), carrots, beef, onion, bay leaves, Worcestershire sauce, and enough water to just cover everything.
Simmer Low and Slow: Cover and simmer for 3–4 hours, stirring occasionally, until the beef is fork-tender and the gravy is rich.
Finish It Off: Stir in the balsamic vinegar and sugar. Mix the cornstarch slurry and add it to the pot to thicken the gravy to your liking.
Serve: Spoon everything generously over mashed potatoes and enjoy that cozy, pub-style comfort.