Combine Dijon mustard, steak seasoning and rosemary in a small bowl. Mix well and rub onto each side of your rib roast.
Place the rib roast in a roasting pan, fat side up. Roast 2-1/4 to 2-1/2 hours for medium rare; 2-1/2 to 3 hours for medium doneness.
When the roast is almost done, place all the ingredients for the chimichurri sauce in a food processor. Coarsely chop until blend.
Remove roast when a meat thermometer registers 135°F for medium rare; 145°F for medium doneness. Transfer roast to carving board; tent loosely with aluminum foil. Let stand 15 to 20 minutes. (Temperature will continue to rise about 10°-15°F to reach 145°F for medium rare; 160°F for medium.)
Carve roast into slices and serve the roast with the chimichurri sauce.