Saute the lamb in a heavy casserole until it is browned on all sides. Remove to a plate. Add the onion and garlic to the casserole and saute until the onion softens and begins to caramelize. Add the curry powder and saute until fragrant, about a minute. Add the tomatoes, sweet potatoes, and chicken stock, and return the lamb to the stew. Cover and cook on low heat for an hour until the lamb is tender. Add the spinach and cook until the greens are wilted. If the sauce needs thickening, add a slurry of cornstarch and water at the finish.