- With the rack in the middle position, preheat the oven to 180 °C (350 °F).
- In a large ovenproof pan, brown the meat in half the oil. Season with salt and pepper. Set the meat aside on a plate.
- In the same pan, brown the onion. Add the garlic and cook for 1 minute. Return the meat to the pan and add the cider. Bring to a boil. Cover and roast in the oven for 1 hour and 30 minutes.
- Meanwhile, in a large skillet, brown the celery root and celery in the remaining oil. Season with salt and pepper. Add the vegetables around the roast and cook covered for about 30 minutes. Add the green onion and cranberries. Adjust the seasoning. Serve with mashed potatoes.