Preheat oven to 350°F. Place crown roast on a rack set in a large roasting pan. Rub roast all over, inside and out, with olive oil, and then sprinkle it with sage, thyme, salt and pepper. Peel onion and push it all the way into center of roast.
Wrap ends of bones with foil to keep them from burning. Pour broth into roasting pan. Roast for 1 hour, then baste with pan juices every 30 minutes. After 2 1/2 hours, using tongs, remove onion from center of roast and place it in roasting pan.
Continue to roast for 1 hour to 1 1/2 hours, until a meat thermometer stuck into meatiest part of a chop registers 160°F. Remove from oven, place roast on a serving platter, tent with foil and allow to rest for 20 minutes.
To serve, remove string holding roast together and carve between chops.
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