Heat 2 tablespoon elderflower syrup to lukewarm. Squeeze out the gelatine and dissolve in it while stirring. Mix with 3 tablespoons of yogurt, then fold in the rest of the yogurt. Whip the cream until stiff and fold in. Chill the mousse for approx. 2 hours.
Possibly set aside some nice raspberries for decoration. Let the raspberries thaw slightly, then heat them with 1 tablespoon elderflower syrup and powdered sugar in a saucepan or in the microwave and mash them.
Using a wet tablespoon, peel off the mousse and serve with the hot raspberries. Decorate with raspberries and possibly blueberries and lemon balm.