Preheat the oven to 375 degrees F.
Place the onions, celery and carrots along the bottom of a large roasting pan. Set the brisket, fat-side up, on top of the vegetables. Sprinkle with 1 tablespoon each salt and pepper. Pour 3 quarts water into the pan.
Roast for 10 minutes. Reduce the oven temperature to 300 degrees F and continue roasting until the brisket reaches an internal temperature of 155 degrees F, about 3 1/2 hours for an 8-pound brisket (20 to 25 minutes per pound).
Let rest for 20 minutes before slicing and serving.
Recipe courtesy of Sherman's Deli & Bakery, Palm Springs, CA