Season the chicken thighs with salt and black pepper, then place them, skin side down, in the pan, and let them cook until the skins turn pale gold in colour. Turn the thighs over and lightly brown the other side.
Peel and roughly chop the onions.
Remove the thighs to a plate and add the onions to the pan, with a splash more oil if needed. Let them cook over a moderate heat for a good 10 minutes until they are starting to soften.
Let the mushrooms cook, adding a little more oil if you wish, until they are glossy, then pour in 500ml of cider.
Let the liquid come to the boil, then lower the heat to a simmer, return the chicken pieces to the pan and partially cover with a lid.
Leave the chicken and vegetables to simmer for about 35 minutes, until the chicken is tender. Check and correct the seasoning as you wish.
Chop a couple of tbsp of parsley and stir into the juices.
Serve in a bowl with plenty of juice from the pan.