Heat olive oil in a large Dutch oven over medium heat. Season beef generously with salt and pepper. Brown the brisket on all sides. Remove from pot to a plate.

Add onions and cook, stirring occasionally, for 4 minutes. Add garlic and cook, stirring, for 30 seconds. Add water, ketchup, vinegar, brown sugar, onion powder, and garlic powder. Raise heat and bring to a boil. Stir well. Add the brisket back to the pan along with any juices accumulated. Cover the pan, lower heat, and simmer for 2 ½-3 ½ hours, flipping the brisket every 30 minutes. When the meat is tender but not falling apart, take off the heat, let it cool to room temperature, and refrigerate overnight.

Thirty minutes before serving, remove from the refrigerator and scrape off any fat accumulated at the top. Remove the meat and slice. Return to the pot and heat over medium heat for 15 minutes or until heated through. Put the meat on a platter.

To thicken the gravy, mix cornstarch or arrowroot starch with about ¼ cup water. Bring the gravy to a boil, stir in the cornstarch slurry, and boil for about 1 minute or until thickened. Ladle some of the gravy over the brisket and serve the rest on the side. Sprinkle chopped parsley over and serve.