Preheat the oven to 190°C.
Brown chicken pieces in oil in a large ovenproof frying pan for 12 minutes.
Remove chicken and fry chorizo and shallots for 5 minutes until soft.
Stir in flour then gradually add cider and stock while stirring continuously.
Stir in mustard, season, and return the chicken to the pan.
Cover with foil and bake for 40 minutes until chicken is cooked through.
Boil chopped celeriac in water and lemon juice for 15 minutes until tender.
Drain and mash celeriac with butter and single cream over low heat.
Stir double cream into the chicken sauce and serve with mash and beans.