Preheat the oven to 425 degrees F. In a saute pan, heat 1 tablespoon of the olive oil. When the pan is hot, add the leeks. Season with salt and pepper. Saute for 1 minute. Add the garlic and sorrel. Season with salt and pepper. Saute for 2 minutes or until the sorrel has wilted. Add the crawfish. Season with salt and pepper. Saute for 1 minute. Remove from the heat and turn into a mixing bowl, cool. Stir in the cheese. Season with salt and pepper. Lay the lamb on a work surface, butterflied side up. Rub the entire lamb with the remaining olive oil. Season with salt and pepper. Spread the crawfish filling evenly over the center of the meat. Roll the lamb lengthwise, tucking the ends in and tie with butchers twine. Crust the outside of the lamb with the cracked black pepper. Place the roast in a shallow roasting pan and roast for 30 minutes. Reduce the heat to 350 degrees F. and continue to cook for 50 minutes. Remove from oven and let rest for 10 minutes before carving. In a saucepan, combine the demi-glace and rosemary. Bring the liquid to a simmer and cook for 8 minutes. Serve with the lamb.