Add the cider & chicken stock into a saucepan then set over a low heat & leave to get steaming hot.
Place the butter into a large saucepan, set over a medium heat then cook until melted.
Stir the flour into the butter then cook out for 1 minute, stirring constantly.
Next, pour in a third of the warm stock then whisk until smooth. Cook out until thick, whisking/stirring regularly. This will take 2-3 minutes over a medium heat.
Once thick, pour half of the remaining stock into the sauce then continue cooking until it has thickened up again, whisking regularly.
Add in the remaining stock then reduce the heat to medium-low. Continue cooking, stirring/whisking regularly until the sauce has thickened back up for a final time. This step will take 5-10 minutes.The sauce will seem quite thick at this point. It will thin out when we add in the double cream. Next, stir in the double cream, mustard & grated nutmeg, leave to warm through for a couple of minutes then add in the grated cheese, stirring to melt.
Season the sauce with salt & pepper, to taste then take it off the heat. Leave to cool slightly.
In the meantime, chop the ham & turkey into 1 cm chunks. Fold this through the sauce once it has cooled then transfer the pie filling to a shallow dish & chill in the fridge to cool down completely.