In a saucepan or frying pan, add the whisky over high heat. If on a gas hob, tip slightly to set the whisky alight (be careful!) to burn off the alcohol.
Once flames have gone out, add the cream and mustard and stir. Reduce heat to medium low and simmer till the sauce has been reduced by half, or to your desired consistency. (You can always add a bit of stock if your sauce has gone too thick.) Season to taste with salt and white pepper, then finish with a small handful of chopped fresh parsley if desired.