First prepare the berry sauce so it has time to cool before preparing the berry mousse. In a small saucepan add mixed berries, sugar and lemon juice and bring to a boil over medium-high heat, stirring constantly. Simmer for about 10-15 minutes or until thickens. Remove from heat and sieve to remove seeds. Dissolve 2 ¼ tsp (7g) gelatin in cold water and let it swell for about 5 to 10 minutes. Dissolve gelatin in the warm berry sauce. Reserve about ? cup (80g) berry sauce and set aside to cool down until ready to use.
Prepare the crust. Line the sides of a round 8 or 9 inch (20-23 cm) springform pan with an acetate sheet and line the bottom of the pan with parchment paper.
Place the digestive biscuits or graham crackers into the bowl of a food processor and crush until crumbs form. Add melted butter and lemon zest and mix until evenly moistened.
Press the mixture into the bottom of the pan using the back of a spoon. Refrigerate until the filling is prepared.
Prepare lemon yogurt mousse. Dissolve gelatin in cold water and let it swell for about 5 to 10 minutes.
Meanwhile in a bowl mix yogurt with powdered sugar and lemon zest.
Dissolve gelatin over low heat, pour over the yogurt mixture. Mix to combine.
In another bowl mix chilled whipping cream until soft peaks form. Gently fold the whipped cream into the yogurt mixture.
Pour the lemon yogurt mousse over the crust.
Spoon the reserved 1/3 cup berry sauce over the lemon mousse. Freeze for about 15 minutes until you prepare the berry mousse.
Prepare berry yogurt mousse. Add yogurt to the remaining berry sauce and mix to combine.
In another bowl mix chilled whipping cream until soft peaks form. Add berry sauce and mix to combine.
Pour the berry mousse over the lemon mousse and smooth the top.
Refrigerate the cake for 4-6 hours to set.
Decorate the cake with fresh mixed berries, lemon peel slices and dust with powdered sugar. Enjoy!