Remove racks of lamb from refrigerator and form the lamb into a crown, tying with string. Rub with olive oil and chopped rosemary, inside and out. Intertwine rosemary sprigs between rib bones. Set aside, and allow to come to room temperature while preparing stuffing.
In a large pot, bring 9 cups water to a boil in a large, heavy-bottomed pot with a tight fitting lid. Add rice and salt, then return to a boil and cook, covered, until tender—about 30 minutes, or up to an hour or more. The wild rice is cooked when most of the rice kernels have cracked open and the white interior of the kernels is visible. If water evaporates before the rice is done add more water, about 1/2 to 3/4 cup at a time until done.
Heat oven to 350°F. In a large sauté pan set over medium heat, melt butter. Add onions and celery and cook until translucent, about 3 to 5 minutes. Toss in grapes, almonds, and thyme. Cook until heated through, about 5 minutes. Add sherry and simmer for 1 minute. Stir in rice, and season with salt and pepper.
Place rack in roasting pan and cover with foil. Transfer roast to roasting pan, and fill with rice stuffing; mound it high, but do not pack too tightly. (Cook extra stuffing in a buttered baking dish covered with buttered parchment paper until thoroughly heated, about 15 to 20 minutes.) Roast for 45 to 60 minutes, or until the meat reaches an internal temperature of 145° to 150°F for medium-rare meat. Turn off oven, and allow roast to rest in oven with door ajar for 10 minutes before serving.
Decorate tips of rib bones with papillotes and garnish platter with bunches of grapes and rosemary sprigs, if desired. To carve, remove papillotes and scoop out stuffing; transfer to a platter. Slice down between bones with a sharp knife, and place on platter. An alternative method is to separate each rib section, lay it flat on a cutting board, and slice between the bones.