Set the oven to 160°C/325°F/Gas Mark 3. Heat a large frying pan and add the chicken thighs, in 1 or 2 batches, skin-side down. Cook until the skin browns well, then turn them over and cook for another couple of mins. Take them out and set aside.
Drain off all the fat from the pan. Add the oil and when hot, add the carrots and leeks and cook for about 5 mins. Spread the veg out in a large baking dish or roasting tin. Add the sprigs of thyme and parsley. Season well. Put the chicken thighs on top. Season again and pour in 600ml boiling water – enough to almost cover the meat. Cover with foil or a lid. Cook for 30 mins, take off the foil and cook for a further 20 mins.
Pour the stock into a jug. Cover the chicken with foil to keep it warm. Bring 300ml of the stock to the boil in a small pan and reduce it by half. Stir in the cream or crème fraiche, whisk well and simmer for a few minutes. Add the mustards and seasoning. Serve the sauce with the chicken, along with potatoes, beans and peas, if you like.