Put the chicken in a large pot. Halve 1 onion, 1 celery stick and 1 carrot. Add to the pot with the herbs, peppercorns and a sprinkling of salt. Add water to come halfway up the chicken, bring to the boil, then cover tightly and simmer for 1½ hrs. Cool slightly, remove the chicken to a dish, then strain the stock into a bowl.
When the chicken is cool enough to handle, strip the meat from the bones and tear into pieces with your hands.
Chop the remaining onion, and cut the celery and carrots into thick slices. Heat the butter in the same pot, add the onion and lardons, then gently fry for 5 mins until just starting to brown. Add the remaining veg, then fry for 2 mins. Stir in the flour, then cook for 1 min. Measure 900ml stock (if you don’t have enough, make it up with water), then gradually add to the pan, stirring. Cover, then simmer for 20-25 mins until vegetables are tender.
Return the chicken to the pan with the mustard and crème fraîche, then return to a simmer, stirring gently. Season and sprinkle with parsley.