Heat a little oil in a large deep shallow pan and fry the chicken all over until browned. Scoop out then add the leeks and fry until they soften. Stir in the mustard and stock and bring to a simmer. Tip in the butter beans and stir. Sit the chicken thighs skin-side up on top. Cover and simmer for 25-30 minutes. Scatter over the parsley before serving.