For the sorbet, mix quark and 75 g sugar. Gradually add 75 g cream and 2 tablespoons of milk. Wash the lemon hot, dab dry and finely grate the peel. Halve the lemon and squeeze the juice.
Stir lemon peel and lemon juice into the quark mixture. Chill the mixture for about 10 minutes, then freeze in the ice-cream maker.
For the pistachio mousse, mix 200 g cream, 100 ml milk and pistachio cream. Pour into a siphon bottle, screw tight and charge with gas cartridges. Keep cold until use.
Peel the pineapple and cut out the hard stalk. Cut the flesh into small pieces. Heat butter in a large pan and add pineapple. Sprinkle with 3 tablespoons of sugar and simmer until the sugar is completely dissolved.
Meanwhile wash tarragon, shake dry, pluck leaves from the stems and chop coarsely. Halve the orange and squeeze the juice. Add tarragon to the pineapple and deglaze with orange juice. Arrange pineapple and mousse on plates.
Shape the sorbet into balls and serve with it.