Rôti de cochon à la moutarde de Dijon

• 1.25 kg (2 lb 12 oz) rolled pork loin
• 50

Heat the butter in a flameproof casserole dish. Add the pork and brown on all sides over medium heat. Add the spring onion and the leek. Add half the wine and season with salt and pepper. Reduce to a low heat, then cover and cook for 1 hour, making sure there is always some liquid in the dish. Add the turnip, potatoes and the rest of the wine, and cook for a further 20 minutes, covered.