The day before, put the butter, sugar and 500ml water in a deep frying pan. Heat until the butter is melted, then bring the liquid to the boil and simmer until lightly syrupy. Drop in the parsnips and cook, covered, for 6-8 mins until just tender then, using tongs, transfer to a roasting tin. Repeat with the carrots then tip them, with the liquid, into the roasting tin with the parsnips. Leave to cool, then put in the fridge overnight.