Prepare you grill: Pour ten pounds of charcoal into the drum and light it.
Meanwhile, wash your brisket clean of all blood and drizzle the fat side with a little olive oil. Sprinkle with garlic powder, onion powder, salt, pepper, and celery salt. Use as much you like; you cannot overdo it. Next, turn the brisket over and cover the meat side with the same ingredients.
After you have allowed your charcoal briquets to turn white, put the fat side of the brisket down on the grill and close the lid with only a crack opening on the intake and exhaust. This will allow the meat to seal in the smoke flavor. Let cook 30 minutes.
Turn the brisket over, then close lid again. Continue turning every 30 minutes for a total cooking time of 4 hours, allowing the heat to die down naturally.
Preheat your oven to 350 degrees. Take the brisket and cut it into half-inch slices and place in a large roaster. Cut your onions into quarter inch slices and place between the sliced brisket. Add 2 cups of water and seal the top with foil, not a lid.
After 2 hours check your brisket, but don’t eat too much. It is not ready yet.
Now, pour one bottle of barbecue sauce over the entire brisket and re-seal and place back in the oven for about 1 hour. The brisket should begin to fall apart at this time. If it doesn’t, put back in the oven for another hour.
When the brisket is cooked through, you can either serve it in slices or chop it up for buns. Before serving, dilute one half of the other bottle of bar-b-que sauce and pour over the meat and put the remaining sauce on the table.