Heat the oil in a large pan and brown the chicken on all sides, then strain the oil into a large oven-proof casserole.
Brush the chicken well with the mustard and place in the casserole. Add the roughly chopped carrots and onions, cover with a lid and transfer to the oven for about one hour. Test by pricking the thickest part of the leg to see if the juices run clear.
Meanwhile melt the butter in a separate pan and add the finely diced carrot, onions and leeks. Sweat slowly without colouring them.
When the chicken is cooked, remove from the casserole and leave to rest. Add the stock, white wine and vinegar to the juices and vegetables in the casserole and cook for 5 minutes.
Now strain the casserole juices over the diced cooked vegetables in the pan and bring to the boil. Check for seasoning, and shake in the parsley, tarragon and chives.
Put in the chicken and pour the broth over before serving.