Place the potato, milk and a pinch of salt in a Thermomix*, set the temperature to 90°C (194°F) and cook on the slowest speed for 35 minutes. Add the butter and mustard and blend on high speed until smooth. With the motor running, slowly add the mustard oil. Season again and pass through a fine sieve. Stir in the whey and store at room temperature until needed.
When ready to serve, gently reheat.
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