Preheat oven to 300°F. Rinse and pat dry the brisket, place it fat side up in a roasting pan, sprinkle on the spice packet, and add the water around it. Cover tightly.

Bake covered for 2 1/2 hours, until a fork twists easily in the thickest part. Check at 2 hours and add time if needed.

Mix brown sugar, mustard, and vinegar. Pour off the cooking liquid, raise oven to 400°F, brush half the glaze on the brisket, and bake uncovered 15 minutes. Brush on remaining glaze and broil 3-5 minutes until caramelized.

Rest the brisket under foil for 10 minutes, then slice thinly against the grain and serve with pan glaze.