Spritz the olive oil in a large saucepan and cook the pancetta until it releases its natural oils. Remove with a slotted spoon and set aside. Add the chicken thighs and cook for 4-6 minutes until well browned all over. Remove the thighs and set aside.
Add the onion, carrots and celery to the pan, season, and cook for another 3-4 minutes until just tender. Add the mushrooms, turn the heat up and cook for 3-4 minutes until golden. Stir in the flour and cook for a minute, stirring until a paste has formed.
Stir through the herbs and pancetta, then pour in the wine. Leave it bubbling for 2-3 minutes until the alcohol has evaporated. Add the stock and peeled shallots, then bring to the boil. Place the chicken thighs back in the pan and cook with the lid on for 10 minutes. Take the lid off and continue cooking for 20-25 minutes until the sauce has thickened slightly and the chicken is cooked through.
Meanwhile, make the mash. Lightly spritz a saucepan with the oil and cook the garlic for 1-2 minutes until fragrant. Add the celeriac and cook for 1 minute, before pouring over cold water to cover. Bring to the boil and cook for 8-10 minutes until very tender, then mash well with seasoning.
To serve, spoon the chicken on to a bed of mash and sprinkle with parsley.