On cutting board, mix thyme with garlic; sprinkle with salt. Holding knife blade at very low angle, rub many times to form smooth paste. Mix pepper and cayenne; rub over roast. Wrap in plastic wrap and refrigerate for 4 hours. (Make-ahead: Refrigerate for up to 24 hours.)
Scatter onions and rosemary in greased roasting pan. Place roast, bone side down, on onion mixture. Roast in 325ºF (160ºC) oven until meat thermometer inserted in centre registers 140ºF (60ºC) for rare, 1 3/4 to 2 hours, or 150ºF (65ºC) for medium-rare, 2 1/4 to 2 1/2 hours, or to desired doneness. Transfer to cutting board or warmed platter; tent with foil and let stand for at least 10 minutes or for up to 30 minutes.
Meanwhile, skim fat from pan juices; place pan over high heat. Add wine; cook, scraping up any brown bits from bottom of pan, until reduced by half, about 3 minutes. In small bowl, whisk stock into cornstarch; whisk into pan and bring to boil. Boil for 1 minute; stir in butter. Strain into warmed gravy boat.
Slice bones off meat and separate each rib. Slice meat and arrange on platter along with ribs. Stir accumulated juices into sauce.