Using your hands, form the boneless rib roast into a cylinder shape. Then, tie the roast with butcher’s twine in order to hold the shape.
In a mixing bowl, combine the salt, pepper, garlic and chopped rosemary. Mix well and set aside.
Place the roast on the rotisserie spit, and secure with forks. Then, evenly coat the roast with dijon mustard. Season the dijon covered roast with the seasoning mixture, ensuring the ends and entire roast are evenly coated.
Place the roast on the preheated grill, and turn on the rotisserie motor.
While the roast is spinning, combine the ingredients for the butter sauce in a small saucepan. Place over medium low heat and whisk till the butter and tallow melt together. Keep warm and set aside.
Allow the roast to spin until the internal temperature reaches 100°F. Then, start basting the roast with the butter sauce every 5-7 minutes until the internal temperature is 5°F less than desired temperature.
Remove the roast from the grill, tent with aluminum foil and allow it to rest for 10 minutes.
Remove the roast from the spit, cut and remove the butchers twine, slice meat and serve with garlic mashed potatoes.