Let the berries thaw for about 15 minutes. In the meantime mix quark and yoghurt, season with sweetener. Roast the muesli in a pan without fat until golden brown. Remove from the pan and let it cool down on kitchen paper.
Lightly mix berries and quark yoghurt. Layer quark yoghurt and muesli alternately in a glass. Serve decorated with fresh berries if desired.