Preheat oven to 450˚F.

Heat olive oil in a large Dutch oven over medium-high heat. Sprinkle roast liberally with salt and pepper, and when oil is hot, place roast in the pan. Sear until brown on both sides. Transfer to a plate; set aside. Add the onions to the Dutch oven and sauté about 4 minutes, stirring often, until golden. Stir in the garlic and cook for another minute. Pour the beef broth into the pot and bring to a simmer. Return the roast to the pot. Cover, and cook in the oven for 1 hour. 

Reduce oven heat to 350˚F. Pour the stout over the roast, and cover again. Return to the oven and cook another 1 hour 15 minutes. Remove the pot from oven and arrange the carrots and potatoes around the roast. Cover again, and cook another 45 minutes. After 3 hours, the roast should be fork-tender. Remove from oven, and transfer the roast and vegetables to a serving platter. Loosely cover with aluminum foil and set aside while you prepare the gravy.

In a small bowl, combine the cornstarch and water to make a slurry. Place the roasting pan on stove top over high heat, and stirring constantly, slowly pour the the slurry into the pan of roasting juices. Cook 3-4 minutes. Serve at the table with the pot roast and vegetables.