Trim any excess fat from the thigh fillets, then wrap each one in a thin rasher of pancetta or bacon, securing with cocktail sticks if necessary. Heat 1 tablespoon of the oil in a large flameproof casserole dish and add the chicken, join-side down, and brown for 2–3 minutes. Turn the chicken to brown the other side, then remove to a plate.
Add the remaining oil to the casserole dish and gently fry the garlic, onion, carrots, fennel or celery, and turnip, stirring a couple of times, until softened. If you need to moisten, add the water, not extra oil.
Add the herbs, red wine, redcurrant jelly and brandy, if using, and simmer for 5 minutes. Stir in the stock and bring to the boil.
Meanwhile, blanch the pickling onions or shallots in boiling water for 2 minutes, then cool and peel. Add to the casserole dish with the chicken thighs and season to taste. Cover and cook very gently for about 30 minutes.
Add the mushrooms, stir well and cook for a further 15 minutes. Add the cornflour mixture and simmer for about 2 minutes until the sauce has thickened slightly. Remove from the heat and allow to cool for about 10 minutes. If you used cocktail sticks, remove these, then serve, sprinkled with the parsley.