Preheat the oven to 300 degrees F.

Season generously with salt and pepper. Preheat a very large skillet over high heat. Add the oil and heat until very hot. Add the brisket and cook, turning once, until well-browned, about 7 minutes the first side, and 5 minutes the second side.

Coat the inside of a large Dutch oven with a tight-fitting lid with the butter. Arrange the onions in an even layer on the bottom of the pot.

Lay the brisket on the onions. Pour in the water, and brush the mustard on the top of the meat. Bring to a boil on the top of the stove, transfer pan to the oven, and cook, covered, basting often, for 1 and 1/2 hours. Turn the brisket over and cook, covered, basting often, for another hour. Turn the brisket over again and cook uncovered, basting often, until well-browned and very tender, about 1 hour and 15 minutes more.

Transfer the brisket to a cutting board and cover with foil to keep warm. Remove the fat from the cooking liquid with a spoon. Transfer the cooking liquid to a medium saucepan.

Using a sharp knife, thinly slice the brisket against the grain and transfer to a large serving platter. Reheat the cooking liquid and pour it over the sliced brisket. Serve immediately.