Heat the oil in a large pan over a medium heat and cook the leek, carrots and chicken thighs for 5 minutes.
Add the rice and chicken stock to the pan. Cover and simmer for 10 minutes.
Stir in the mushrooms and Dijon mustard, cover and cook for 15 minutes until the rice is tender and all of the stock has been absorbed. Cut into a thick portion of chicken and check that it is cooked through, with no pink showing and the juices run clear.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.