Heat oven to 450 degrees. Season beef generously with salt and pepper. Heat oil in large dutch oven over medium-high heat for 2 minutes. Place roast in pan, and sear well on both sides until nice and brown. Remove roast to plate and set aside. Add onions to the dutch oven and cook, stirring often, until the onions are starting to brown on the outsides. About 4 minutes. Add garlic and cook for an additional minute. Add the beef stock to the pan, then add beef to the pan with the stock. Cover, and cook for 1 hour.
Reduce oven temperature to 350. Remove the roast from the oven, add the stout, re-cover, and cook for an additional 1 hour 15 minutes. Remove roast again, add the vegetables and re-cover. Cook for an additional 45 minutes.
To make the gravy, remove the roast (and, it will be falling apart!) and place on a platter. Then, remove the various vegetables and arrange them on the platter as well. Cover it with aluminum foil to keep warm while you make the gravy. Using a fine mesh strainer, pour the braising liquid into a medium saucepan. Combine the cornstarch and the cold water in a small dish, then add to the gravy. Cook over medium heat, stirring constantly, until the cornstarch works it’s magic and the gravy thickens.
You can try to slice the pot roast, but I just like to use two forks to pull it apart into large chunks. Whatever floats your boat, though!