Remove roast from refrigerator; stand at room temperature 2 hours before cooking.
Preheat oven to 450°F. Mix salt, pepper, garlic powder, onion powder, thyme, rosemary, and cayenne in a small bowl.
Rub entire roast with oil; coat generously with seasoning, pressing into meat.
Place bone-side down in roasting pan; roast at 450°F for 15 minutes to form crust.
Reduce oven to 325°F; roast 13-15 minutes per pound for medium-rare (about 1.5-2 hours total), until internal temperature reaches 120-125°F.
Remove; tent loosely with foil and rest 20-30 minutes before carving. Temperature will rise to 130-135°F while resting.
Slice between bones; serve immediately with horseradish sauce.