Rinse the chicken then wipe dry. Rub with salt and pepper.
Heat the butter in a frying pan.
Arrange the chicken thighs in the hot butter, skin side down in a single layer. Cook over high heat until the skins are nicely browned, about three minutes. Flip and brown the other side (another two minutes).
Add the onion slices, stir and cook just until the softened a bit. Pour in the chicken broth. Lower the heat, cover and cook for about 20 minutes until the chicken thighs are almost done and the mixture is almost dry.
Add the mushrooms and peas. Sprinkle with more salt and pepper. Stir. Cover and cook for two minutes.
Pour in the mushroom soup. Cook until the condensed soup has thinned out and has the consistency of a thick sauce.
Stir in half of the parsley.
Transfer the chicken, mushrooms, peas and sauce to a serving platter (a shallow bowl is a better idea). Sprinkle in the remaining parsley and serve.