How to Make It
Preheat the oven to 300°. Season the brisket generously with salt and pepper. In a large enameled cast-iron casserole, spread the onions in a single layer and lay the brisket on top. In a small bowl, whisk the ketchup with the tomato paste, soy sauce, brown sugar and garlic. Spread the sauce all over the brisket.
Cover the casserole and transfer to the oven. Braise the brisket for about 3 hours, until the meat is very tender. Uncover and continue cooking for 1 hour, until the sauce has thickened. Let the brisket cool slightly in the sauce for 30 minutes.
Transfer the brisket to a work surface and slice across the grain. Arrange the slices on a platter and spoon some of the sauce over the top. Garnish with cilantro and serve with the remaining sauce on the side.