Soak gelatin. Drain the pineapple (catch the liquid in the process).
Beat the sugar, egg yolk and 6 tablespoon pineapple juice to a thick white cream. Squeeze out the gelatine and dissolve it. Stir evenly into the cream. Rub the lemon zest into the cream. Cut half of the pineapple slices into pieces and fold them into the cream. Put in a cool place.
Beat the quark and crème fraîche until smooth. Whip the cream. As soon as the cream becomes thick, fold in the quark mixture first and then the cream. Let it set in the refrigerator.
Cut the remaining pineapple slices into triangles and garnish the quark cream with them before serving.