Preheat oven to 425ºF. After the roast has been sitting at room temperature, pat dry with paper towel; season with salt, pepper and mustard. Heta an oven-proof pan over medium-high and add oil. When hot, sear roast on all sides and ends. Place a rack with the fat side up. Insert a probe, and transfer to the preheat oven; cook for 20 minutes. Reduce heat to 325ºF and cook until internal temperature registers 140ºF.
While roast is cooking, in a medium bowl combine herbs and garlic. Pour grapeseed oil, stirring, until the herb mixture is moist. When roast internal temperature reaches 115ºF, remove from the over and quickly spread on top, pressing down lightly. Return roast to oven to finish cooking.
Remove roast from oven and place on a carving board. Tent the roast with aluminum foil and let it rest for 20 to 30 minutes before slicing. If it’s too rare for some, quickly sauté both side in a hot pan with a little gravy for up to 2 minutes.
Note: Bone-in prime rib will take more time to cook in the oven.