Mix the spice rub ingredients together in a large bowl. Massage the spice rub into the beef and cover with plastic wrap.
Set up and light your Smokey Mountain Cooker as per the instructions in your handbook. Once the smoker has reached the desired temperature, add 3 chunks of Weber Firespice Mesquite wood through the front door. Close the door.
After 10 minutes, smoke should be visible coming from the top vent. Place the beef on the cooking grill and close the lid.
Cook the beef for 8 to 10 hours or until the internal temperature has reached approximately 95°C. Note: For longer cooking times like this, remember to check the temperature of the smoker every hour and ensure the water pan does not run dry.
Towards the end of the cooking time, make the sauce. Place the vegetable oil in a saucepan on a side burner or stove top over a medium heat. Add the onion and garlic, cook for a few minutes or until the onion and garlic has softened. Add the remaining sauce ingredients. Cook over medium to high heat for 10 minutes or until the sauce has thickened, stirring regularly. Remove the sauce from the heat and set aside.
Once the beef has cooked, remove from the smoker and wrap in foil. Let the beef rest for at least 30 minutes.
Pull the beef using two forks and mix with the sauce.