Tip the leeks and oil into a flameproof casserole dish on a low heat, add the butter and cook everything very gently for 10 mins or until the leeks are soft.
Put the chicken, skin-side down, in a large non-stick frying pan on a medium heat, cook until the skin browns, then turn and brown the other side. You shouldn’t need any oil but if the skin starts to stick, add a little. Add the chicken to the leeks, leaving behind any fat in the pan.
Add the stock to the dish and bring to a simmer, season well, cover and cook for 30 mins on low. Stir in the mustard, crème fraîche and peas and bring to a simmer. You should have quite a bit of sauce.
When you’re ready to serve, put the grill on. Mix the breadcrumbs and parsley, sprinkle them over the chicken and grill until browned.