Sorry to jump in here but have you tried cooking it like you would cook an eye of the round roast? Preheat oven to 500F cook meat for 7 minutes per pound at 500F then turn oven off and let sit for 2 to 2.5 hours. Do not open oven at all while it sits in there. Take meat out and tent loosely for 10-20 mins under foil. I like to make a rub for the meat and Let it sit with rub and wrapped in plastic for an hour to allow the meat to reach room temp first. You can either put in roasting tray on a rack dry like that. Or you can sear on a hot skillet on all sides for about 3mins per side. Then reduce oven heat to 475F and cook as suggested above for eye of round beef. If lamb has bone in it (not French rack) leave in oven for the 2.5 hours. Bon appetite!
This sounds quite interesting. Can you tell me what cut of lamb you have in mind? By your last comment, it sounds like you are thinking of rack of lamb; however, the procedure you outline sounds like it is appropriate only for a large cut, like a leg or something. Wouldn't the rack get cooked well beyond medium with your method?
Personally, I like to cook my rack of lamb by sous vide at about 125 deg for 10 hours, then cover with a persillade and sear the outside under a broiler. Ohhh, that is tender and goooood!