Fry the onion, leeks and apple in the oil in a casserole or deep pan, until softened. Stir in the sage for 1 minute, and then stir in the flour.
Gradually stir in the stock, followed by the mustard and honey. Bring to a simmer and stir in the chicken and leftover vegetables. Cover and let gently simmer for 15 minutes until the chicken is piping hot.
Preheat the oven to 200°C, fan 180°C, gas mark 6. Place a baking tray in the oven to heat through. Heat the giant Yorkshire puddings for 6 minutes on the hot baking tray.
Heat the mashed potato as per pack instuctions.
Divide the mash between the Yorkshire puddings and spoon over the chicken and veg casserole to serve.
*All information included in this recipe has been provided by Aunt Bessie's.