Preheat the oven to 425 degrees F. Heat 1 tablespoon of the olive oil in a large skillet over medium heat. Add the leeks and season with salt and pepper. Cook, stirring, for 1 minute. Add the garlic and sorrel, and season with salt and pepper. Cook, stirring, for 2 minutes or until the sorrel has wilted. Add the crawfish, and season with salt and pepper. Cook, stirring, for 1 minute. Remove from the heat and transfer the mixture to a mixing bowl. Cool. Stir in the cheese. 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.