Method:
Let roast stand at room temperature for 1 hour. Meanwhile, preheat oven to 450°F.
Place roast on a rack in a roasting pan and rub all over with salt, pepper and thyme. Roast meat, rib side down, for 30 minutes, then reduce heat to 375°F. Continue to roast for 12 to 15 minutes per pound (or until internal temperature reaches 145°F), about 2 1/2 hours.
About 30 minutes before the roast is done, brush onions all over with oil and sprinkle with sage, salt and pepper; arrange on a baking sheet. Place in the oven with the roast. When roast is finished, remove and let stand for at least 20 minutes before carving. Meanwhile, increase oven temperature to broil, move onions to a higher rack, and broil until tops are slightly charred and caramelized. Arrange roast and onions on a platter and serve.