Put the potatoes and milk into a medium pan, add water to just cover, season, then bring to the boil. Simmer for 15 mins or until tender. Add the rosemary a few mins before the end of cooking to infuse. Drain off all but
a few tbsp of the cooking liquid.
Roughly crush the spuds, fold in the truffle oil and cream, then tip into a small baking dish, adding a few rosemary leaves to decorate. Flash under the grill for a few mins to colour the top. If making ahead, reheat in the oven at 200C/fan 180C/gas 6 for 15 mins until just bubbling and starting to turn golden.