Heat the oil in a wide casserole dish over a medium-high heat, and season the chicken all over. Cook the chicken on all sides until the skin is crisp and golden brown (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 for 5-6 mins until starting to turn bright green and tender. Add a splash of water if the leeks are beginning to turn golden. Scoop out and set aside with the chicken.

Tip the potatoes into the pan, then pour over the chicken stock and nestle in the thyme. Cook, uncovered, over a high heat for 10 mins, then reduce the heat to medium. Stir in the mustard, crème fraîche, peas and leeks. Nestle the chicken back into the pan, so it’s mostly covered by the sauce.

Put on the lid and cook for about 20-25 mins until the chicken is cooked through and the potatoes are tender. If you prefer a thicker sauce, mix the flour in a small bowl with 2 tbsp cold water. Stir into the sauce and simmer with the lid off for 2-3 mins more until the sauce thickens. Grind over some black pepper to serve.