Preheat the oven to 180°C/Fan 160°C/Gas 4. Heat the oil in a large heavy based casserole dish, which will just hold the chicken. Add the bay leaf and cook for a few minutes until starting to brown.
Stir in the leeks, onion and garlic and cook for 8 minutes until the veg have softened. Stir in the flour and cook for just a minute.
Pour in the stock, stirring well, and then add the chicken and bring to a simmer. Cover and cook in the oven for 45 minutes then remove the lid and cook for a further 30 minutes until golden. Add the Dijon mustard and pour the cream around the outside of the chicken. Cook in the oven for 10 minutes.
Remove the chicken from the casserole dish and place on a board to carve.
Serve with mashed potato and spoon over the creamy leek sauce.