Thaw and puree the frozen strawberries. Cut the fresh strawberries into small cubes.
Now put the pureed strawberries in a small saucepan on the stove and warm them up slightly. Stir in the instant gelatine and stir until it has dissolved. Sweeten as desired and / or season with lemon juice. Remember to taste a little stronger, because quark and egg white are added.
Stir the fresh strawberries into the puree and put the mixture in a cool place for about 30 minutes.
Take the already slightly set strawberry mousse out of the refrigerator and mix in the low-fat quark. Beat the egg white until stiff and fold in carefully.
Chill the mousse for at least 2 hours and then serve decorated with white chocolate.