Put the caster sugar and water in a small saucepan and simmer gently until the sugar has dissolved. Add the lavender flowers to the syrup, if using, and simmer for 3 minutes. Remove from the heat and leave to cool.
Whisk the egg white in a clean bowl until it stands in stiff peaks. Mix the Quark and crème fraîche in another bowl, stir in the cold strained lavender syrup, then fold in the whisked egg white.
Line 4 perforated moulds or recycled, clean yogurt pots with small holes in the bottom, with damp pieces of muslin and stand the moulds on a plate to catch the liquid that will drip out. Spoon the cheese mixture into the moulds, level, and chill overnight or until required.
To make the fruit syrup, put the caster sugar and water in a small saucepan and heat gently to dissolve the sugar. Add half the fruit to the warm syrup and cook for 1 minute, then remove from the heat. When the syrup and fruit are cold, stir in the remaining fruit. Cover and chill until required.
When ready to serve, turn the cheeses out of their moulds on to 4 serving plates. Serve with the soft fruits and decorate with mint sprigs.