In a very large pot, cover the lamb bones with the water. Add the onion, garlic, thyme and bay leaf and bring to a boil. Add a large pinch of salt and simmer over moderately low heat, skimming the surface occasionally, for 1 1/2 hours. Strain the soup base.
In a medium saucepan of boiling water, add the barley, cover and simmer over moderate heat until the barley is tender, about 50 minutes. Drain.
In a large pot, melt 2 tablespoons of the butter. Add the onion and celery and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes. Add the soup base, split peas and carrots, cover and simmer over low heat, stirring occasionally, until the peas are almost dissolved, about 1 hour. Add the lamb and turnips, cover and simmer until the turnips are tender, about 15 minutes.
In a small bowl, blend together the remaining 1 tablespoon of butter with the flour to make a paste. Whisk about 1 cup of the soup into the bowl until smooth. Whisk the mixture into the soup. Add the leftover Ancho Gravy and barley and simmer, whisking a few times, until slightly thickened, about 5 minutes. Season with salt and pepper and serve.
The soup can be refrigerated for up to 2 days. Reheat gently.
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