Heat the vegetable oil in a large pan over medium heat. Add the onion and sauté for 5-8 minutes, or until the onion is very soft. Add seitan, and cook until lightly browned, stirring often.
Add the barbecue sauce and water and cook, stirring every 5 minutes, until the sauce has thickened and been absorbed by the seitan. Add hot sauce to taste if using.
Toast the inside of the buns to prevent them from getting soggy, then spoon the seitan mixture onto the rolls and garnish. Serve hot.