Bring water to the boil with salt and a bit of butter in a saucepan.
In the meantime, peel the potatoes and cut them into cubes. Then put in the saucepan, cook until soft and then drain.
Heat the milk. Mash the potatoes or press them through a potato press. Add hot milk, pesto and a flake of butter and stir until smooth. Finally, season with salt, pepper and nutmeg.