Heat the butter in a large frying pan. Add the pistachio nuts and cook for a few mins until toasted, turning and shaking the pan frequently to prevent them from burning. Sprinkle over the sugar and stir until the sugar has completely dissolved. Turn out on to a plate and leave to cool (about 5 mins), then roughly chop.
Using a sharp knife, cut the bottom off each orange, then carefully cut away the skin and pith, following the curve of the fruit. Cut into slices. Arrange on a plate with any juices. Serve with the nuts scattered over and the yogurt or crème fraîche to dollop on top.