Preheat the oven 190C/170C Fan/Gas 5. Season the chicken thighs all over with a little salt and lots of black pepper.
Heat the oil in a large non-stick casserole pan over a medium heat and fry the chicken for 7–8 minutes, skin-side down, or until the skin is nicely browned. Turn and cook on the other side for 3 minutes more. Transfer to a plate.
Return the pan to the heat and add the onion, bacon and mushrooms. Fry over a medium-high heat for 4–5 minutes, or until lightly browned, stirring regularly. Add the carrots and flour and toss together well.
Sprinkle with the thyme, then pour in the stock, a little at a time, stirring well between each addition. Add the chicken pieces back to the pan and bring to a gentle simmer. Cover the pan with a lid.
Place in the oven and cook for 45 minutes. Take out of the oven and stir in the leeks.
Return to the oven for a further 15 minutes, or until the chicken and leeks are tender and the sauce has thickened. Serve.
