Carefully remove bones from lamb, except for the last "shin" bone. Scrape meat off bones. Preheat oven to 350 degrees F.
In a large saucepan, add 1/4 cup clarified butter and onions, carrots, and leeks. Stir and add in bones and 2 cups cold water. Raise heat and bring to a boil, then lower to a simmer and reduce by half. Season with sprig of parsley, bay leaves, and crystallized salt.
Heat small frying pan. Add bread crumbs and pour remaining butter on top. Cook gently. Add orange segments and continue to cook. Meanwhile, combine orange rind and 3/4 cup water in small pan and bring to a boil. Add arrowroot and sugar and reduce to low heat until it begins to thicken to a glaze. Transfer bread crumbs and orange segments to a mixing bowl. Season stuffing with Grand Marnier, salt, white pepper, and chopped parsley.
Stuff lamb and close pouch with skewers, lacing up with string in between, like a big boot. Make shallow diagonal slashes over top of lamb. Season flour with salt and pepper. Dust lamb with flour. Place in baking dish and roast for 1 hour 40 minutes. Remove lamb from oven and place in a bain marie (small baking dish set in a larger baking pan which is filled half way with water). Increase heat to 500 degrees F. and cook for another 15 minutes. Baste with orange glaze.