Prepare the smoker for indirect cooking over very low heat (110-130°C).
Trim any excess fat from the lamb.
In a small bowl, combine the salt, pepper, whole mustard seeds, paprika, brown sugar, rosemary leaves and garlic.
Using a basting brush, paint the lamb with mustard and evenly cover with the spice mix.
When the smoker is ready, add wood chunks (we used pecan wood chunks) to the charcoal. Place the lamb on the cooking grill, close the lid and cook over indirect very low heat for 8 to 9 hours or until the temperature has reached 93°C.
If you are short of time, after 5 hours wrap the lamb tightly in alfoil and continue to cook until the internal temperature has reached 93°C (approximately a further 2 hours).
Remove the lamb from the barbecue. Wrap in alfoil if you have not done so already, and leave to rest for at least 30 minutes before pulling with two forks.