Heat the butter and a drizzle of olive oil in a deep, lidded frying pan or heavy-based pan over a high heat. Add the chicken pieces and stir-fry for 1-2 mins. Add the leeks and thyme, stir for a further 1 min, then add the plain flour, stirring into the pan juices.
Gradually add the hot stock, bring up to the boil, then simmer for 2 mins. Throw in the vegetables and cook for a further 2 mins before adding the cream and herbs, if using. Serve with mini jacket potatoes or quick-cook rice.