Heat the oil in a wide shallow casserole dish (with lid) over a medium to high heat, and season the chicken all over. Cook the chicken on both sides until the skin is crisp and golden brown (4 to 5 minutes on each side - they don’t need to be cooked through at this point). Transfer to a plate.
Discard all of the fat in the pan except about 1 tsp, and fry the leeks over a medium low heat for 5 to 6 minutes until they start to turn bright green and tender. Add a splash of water if the leeks are beginning to turn golden. Scoop them out and set aside in a bowl or on a plate.
Tip the potatoes into the pan, then pour over the chicken stock, nestle in the thyme and season with salt and freshly ground black pepper. Cook, uncovered, over a high heat for 10 minutes, then reduce the heat to low. Stir in the mustard, crème fraiche and leeks. Nestle the chicken back into the pan, so it’s mostly covered by the sauce.Put on the lid and simmer for 20 minutes. Then add the peas and cook, covered, for a further 5 minutes until the chicken is cooked through and the potatoes are tender. If you prefer a thicker sauce, remove the chicken onto a plate, increase the heat to high and stir flour and water mixture into the sauce. As soon as it's combined, with no lumps, return the chicken to the pan and simmer with the lid off for 2 to 3 minutes more until the sauce thickens. Grind over some black pepper to serve.
Put on the lid and simmer for 20 minutes. Then add the peas and cook, covered, for a further 5 minutes until the chicken is cooked through and the potatoes are tender. If you prefer a thicker sauce, remove the chicken onto a plate, increase the heat to high and stir the flour and water mixture into the sauce. As soon as it's combined, with no lumps, return the chicken to the pan and simmer with the lid off for 2 to 3 minutes more, until the sauce thickens. Grind over some black pepper to serve.