1. Swirl oil into a large, heavy pot over medium-high. Add lamb, bones, and onions; cook until bottom sides of bones turn dark brown, 8 to 10 minutes. Flip, reduce heat to medium, and add celery and carrots. Continue cooking, turning occasionally, until lamb and vegetables are deep brown on all sides, 20 to 25 minutes. Add 12 cups water, leek trimmings, peppercorns, and parsley stems; season with salt. Bring to a boil; skim foam. Reduce heat and simmer, partially covered, until meat is fork-tender, about 1 1/2 hours. Remove from heat and let stand, uncovered, until fat begins to rise to surface, about 2 hours. Skim fat; strain stock through a fine-mesh sieve, reserving lamb. When lamb is cool enough to handle, trim excess fat, then cut into bite-size pieces and reserve, discarding bones.

2. Place stock in a clean pot; add turnips and barley. Bring to a boil, then reduce heat and simmer, partially covered, until barley is just tender and turnips are easily pierced with the tip of a sharp knife, about 20 minutes; transfer turnips to a cutting board. Add leeks to pot and simmer until tender, 5 minutes. Meanwhile, slice turnips lengthwise into 1/2-inch moons and return to pot, along with lamb and peas. Simmer until heated through. Serve, with chopped parsley alongside.