In a large heavy flameproof casserole, heat the oil over moderate high heat until it is hot but not smoking and in it brown the beef, patted dry, and seasoned with salt and pepper, on all sides. Transfer the beef to a plate, add the onions and the garlic to the casserole, and cook the mixture over moderate heat, stirring, until the onions are golden.
Stir in the tomatoes, vinegar, brown sugar, bay leaf, and water, bringing the mixture to a boil, and add the beef. Braise the beef, covered, in a preheated 350 degree F oven for 1 hour, add the carrots and braise the mixture, covered, for 30 minutes. Add the potatoes, drained, and braise the mixture, covered, for 30 minutes, or until the beef and the vegetables are tender.
Pot roast may be prepared up to this point 2 days in advance, cooled, uncovered, and chilled, covered. Discard any solidified fat before reheating the pot roast.
Transfer the beef with slotted spoons to a cutting board and let it stand for 10 minutes. Skim the fat from the top of the cooking mixture and bring the mixture to a boil. Stir the cornstarch mixture and add enough of it to the boiling cooking mixture, stirring, to thicken the gravy to the desired consistency. Simmer the gravy mixture, stirring occasionally, for 2 minutes, season it with salt and pepper, and discard the bay leaf.
Slice the beef and spoon some of the gravy over it. Serve gravy on the side, as well. Garnish the beef with the parsley.