Preheat oven to 375 degrees F.
Roast the potatoes on a flat sheet pan until almost cooked through, approximately 20 minutes. Allow the potatoes to cool at room temperature for 20 minutes, or until they can be easily handled.
With a paring knife, remove the skins of the potatoes without cutting into the potato itself. Place the peeled potatoes into a bowl. Combine the oil and rosemary. Place half of the mixture into the bowl of potatoes and toss. Season with salt and pepper. Return the potatoes to the sheet pan. Rub the lamb with the remaining half of the oil and rosemary mixture. Season with salt and pepper. In a heated saute pan, or on a hot grill, sear the loins on all sides. Place the potatoes and lamb in the preheated oven (on separate pans).
Combine seasoned rice wine vinegar and minced shallots in a small bowl.
When internal temperature of the lamb is 120 degrees, remove the lamb and let it rest for about 5 to 7 minutes. When lamb has rested, the potatoes will be ready. Toss them in the oil in which they were cooked to prevent dryness and ensure even seasoning.
Pour the juices from the lamb into the bowl containing the shallots and vinegar. Season with salt and pepper. While constantly whisking, slowly pour in 1/4 cup of olive oil. To present, arrange the potatoes on plate, slightly off center. Use vegetables of your choice and place next to potatoes. Slice lamb into 4 equal-sized pieces. Fan the pieces using the bone for height in presentation. Drizzle with vinaigrette.
Purchase lamb from a good source. Have butcher prepare racks in 4-bone portions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Stephen Jalbert, The Grill at the Ritz-Carlton - Dearborn, MI