Marinade chicken, wine, vegetables, and herbs in the fridge for 12 hours.
Preheat the oven to 180°C.
Remove and dry chicken, then strain and reserve the wine and vegetables.
Season chicken with salt and pepper and fry in a casserole until golden-brown.
Brown the reserved vegetables in the same pan, add wine, and reduce by half.
Return chicken to the pan and add enough stock to almost cover the meat.
Cover with a paper cartouche and bake for 35 minutes before resting.
Blend curly parsley with 2.5 litres of water in a food processor.
Line a colander with muslin cloth and fill with ice over a bowl.
Heat parsley water to 80°C, then strain through ice-muslin to collect chlorophyll purée.
Boil the potatoes in salted water until they are soft.
Strain the coq au vin sauce, reduce it by half, and return chicken to it.
Rice potatoes, then stir in butter, milk, and the reserved parsley purée.
Fry onions, pancetta, and mushrooms in oil, then reduce with stock and butter.
Serve chicken and sauce inside a well of parsley mash with the garnish.