Preheat the oven to 325 degrees F. Season the lamb with salt and pepper. Place the oil in a Dutch oven or ovenproof pot set over medium-high heat. When hot, add the lamb and brown on all sides. Add the onion, celery and garlic and cook 3–4 minutes. Mix in the flour and cook 2–3 minutes more. Slowly, stirring constantly, mix in the stock. Mix in the mustard and rosemary and bring to a simmer. Cover and cook in the oven for 45 minutes. Stir in the squash, sweet potato and parsnip and cook in the oven 30 minutes more, or until the meat and vegetables are tender. Stir in the parsley, adjust the seasoning and serve.
Note: If cubed lamb stew meat is unavailable, buy an equal weight of lamb medallions and cut them into cubes.