You will need a spring-form pan. You can use a 9", 7" or make mini cakes with 4" pans. (I used a 7" pan.)
Trace the bottom of your pan on parchment paper. Cut out the circle to line the bottom of the pan.
Using plastic wrap, line the sides of your spring-form pan. Please see photo above for an example.
Next, make the simple syrup. In a small bowl add 1 cup of warm water, 1 tablespoon of sugar and 2 tablespoons of lemon juice. Stir until the sugar is fully dissolved. Make sure your bowl is big enough to dip the ladyfinger cookies in.
The ladyfinger cookies need to be trimmed to fit correctly (flat end down) in the spring-form pan. Using a serrated knife cut off about 1 inch of the bottom of the cookie. You will see a slight indent towards the bottom of the cookie and that is where you will trim the cookie. Save the end cookie pieces to be used for the bottom crust. Please see the photos above for an example.
Dip the rounded sugared side of the ladyfinger cookie into the simple syrup and then place upright in the spring-form pan with the flat side touching the side of the pan to start forming the circular cookie crust. You will dip each ladyfinger cookie into the simple syrup. Please see the photo above for an example.
Line the bottom of the pan with the dipped ladyfinger cookies. Place the cookies flat-side facing out touching the sides of the pan. Also, dip the trimmed pieces in the simple syrup and use the pieces to fill in the spaces between the cookies in the bottom of the pan. Please see the photo above for an example.
Boil 1 cup of water in a glass measuring cup in the microwave on on the stove top. Add 1 packet of Knox gelatin to the hot water and stir until it is fully dissolved. Allow the mixture to cool but not to become solid.
In a large bowl using a standup mixer or hand mixer start to beat heavy whipping cream, 1/2 cup of lemon juice and powdered sugar on medium speed until stiff peaks form.
Add the cooled liquid gelatin mixture to the cream and beat until thick.
For the first layer, pour half of the lemon mousse into the ladyfinger-lined pan.
For the second layer add fresh berries. I recommend blueberries, blackberries, and raspberries.
For the final layer pour the rest of the lemon mousse on top of the berries and smooth the top with a spatula or back of a spoon.
Cover the cake with plastic wrap and refrigerate for at least 4 hours and firm. I highly recommend refrigerating overnight for best results.
To serve, remove springform walls, plastic wrap and top with more berries. I recommend that you start with placing your largest berries first on the cake and then filling in the rest of the cake with the smaller berries. I started with the strawberries, then added the blackberries, then the raspberries and filled in the rest with the blueberries. I also added fresh mint leaves as a garnish too.
The cake can be covered and stored for several days.