Mix the quark, vanilla seeds and lemon zest, fold in the whipped cream.
Beat the egg whites with the salt until stiff. Add the sugar little by little and continue beating until the egg whites turn glossy. Fold the egg whites into the quark mixture, divide into glasses, refrigerate for approx. 2 hrs.
Slice off the top and bottom of the oranges, then peel all round the fruit down to the flesh. Using a sharp knife, cut out the segments between the white membranes, arrange on top of the quark mousse. Sprinkle with pistachios.