Rinse the chicken (inside and out) in cold water, pat dry.
Coat the chicken with mustard, season.
Chop the celeriac and onions. Heat the oil in the cooking pot. Sauté the celeriac and onion for approx. 2 mins.
Add the bay leaf and mustard, pour in the milk and stock and bring to the boil. Add the chicken.
Braise for approx.
1 3/4 hrs. in the lower half of an oven preheated to 180°C. Remove from the oven, remove the chicken and discard the bay leaf. Puree the sauce, season with salt.
Chop the potatoes, cook in boiling salted water for approx. 15 mins. until soft. Drain the potatoes well.
Return the potatoes to the pan. Add oil and mash with a potato masher or fork, season with salt.
split the chicken into pieces and serve with the sauce and the mashed potato.