- Heat a large saucepan with the oil and butter, add the chicken and cook gently for 4-5 minutes until coated all over.
- Season lightly at this stage with some salt and pepper and then add in the carrots, celery, leeks, garlic and onion and continue to cook for a further 3 or 4 minutes.
- Sprinkle in the plain flour at this stage, which will cause the mixture to look quite unpleasant – don’t panic!
- Add in the milk and the chicken stock along with the thyme sprigs and stir continuously until the mixture begins to thicken.
- Next add in the wholegrain mustard, place a lid on the saucepan, reduce the heat to a gentle simmer and simmer for approximately 20-25 minutes until both the chicken and vegetables are cooked through.
- Stir occasionally during the cooking process to prevent the mixture from sticking at the bottom.
- Garnish, just before serving, with some freshly chopped parsley.
- Cook the rice according to the packet instructions and serve immediately.
Top tip for leftovers
As this is a large recipe, any remaining casserole can be stored in the freezer for a later date.