Halve the melon and remove the seeds with a spoon. Cut 12 melon balls from one half with a ball cutter. From the other half cut off a slit, cut the flesh from the skin and slice.
Remove the remaining melon flesh from the skin and dice. Mix approx. 50 g muesli, up to 1 tablespoon, and 4 tablespoons of melon cubes and divide into 4 glasses. Whip the cream until stiff. Stir quark, sugar, vanilla sugar and milk until smooth.
Fold in cream and remaining melon cubes and spread on the muesli. Sprinkle with remaining muesli and decorate with melon balls and slices. Serve garnished with lemon balm.