Preheat the oven to 300 degrees F (150 degrees C).
Place the brisket in the center of a roasting pan. Arrange the potatoes and carrots around the sides. Place the onion and garlic on top of the roast, and season with salt and pepper. Empty the seasoning packet over the roast, and pour enough water into the pan to almost cover the potatoes. Cover with a lid or heavy aluminum foil.
Roast for 5 to 6 hours in the preheated oven, until the roast is so tender it can be flaked apart with a fork.