Remove brisket from the refrigerator far enough ahead of time to allow it to come to room temperature.
Preheat oven to 275°F.
In a large Pyrex baking dish (or very heavy metal pan), thoroughly mix the barbecue sauce, Worcestershire sauce, liquid smoke, garlic powder, celery salt, lemon pepper, salt, chopped onion, and water to make sauce.
Put the brisket in the baking dish, and turn it over once to coat it with the sauce.
Seal the dish tightly with heavy-duty aluminum foil.
Bake at 275°F for 5 to 7 hours (about 1 hour and 15 minutes per pound).
Remove from oven and allow to stand for 1/2 hour before slicing.
Carve across the grain in thin slices/bite sized pieces.
Place carved meat back into the sauce and stir until well coated.
Serve in warm tortillas sprinkled with Jack cheese, red onion, and cilantro.