In a food processor, combine the pistachios, butter, and icing sugar. Arrange the nectarines on a baking tray and squeeze lemon juice over them. Place spoonfuls of the pistachio mixture over the nectarines, reserving half the mixture.
Bake for 10-15 minutes or until the fruit is tender and the topping is crisp. Cool for 5 minutes before serving. Top each serving with a little of the remaining pistachio mixture. Serve with yoghurt or crème fraîche.