How to Make It
In a large enameled cast-iron casserole, heat the 3 tablespoons of oil until shimmering. Season the lamb with salt and pepper. Working in 3 batches, cook the lamb over moderate heat until browned all over, about 8 minutes per batch. Transfer to a large plate. Add the onions to the casserole and cook over moderate heat, stirring, until golden, about 5 minutes. Add the garlic and cook, stirring, until golden, about 2 minutes; transfer to the plate.
Remove the casserole from the heat and add enough oil to make 6 tablespoons of fat. Whisk in the flour, then return the casserole to the heat. Add the wine and bring to a simmer over moderate heat, scraping the bottom of the casserole. Stir in 4 cups of water along with the stock and whisk until smooth and simmering, then add the lamb and onion mixture and bring to a simmer. Cover and cook over moderately low heat, stirring occasionally, until the lamb is tender, about 1 hour and 45 minutes.
Add the turnips, carrots and potatoes to the casserole and cook until tender, about 30 minutes. Stir in the heavy cream; season with salt and pepper. Ladle the stew into bowls and garnish with chopped parsley and mint. Serve with crusty bread.