Preheat an oven to 180 Celsius degrees (160 degrees for fan ovens).
Put a piece of baking paper on the bottom of a removable-bottom 18cm diameter cake tin (no need to cut a disc out, the edges can stick out as it will be easier to take the sponge out).
Put the egg whites into a big bowl, whisk them with an electric mixer till stiff, slowly add the icing sugar and beat a while longer.
Mix the egg yolks with a fork (just to break them), add to the whisked whites and gently stir.
Gradually add the flour mixed with the baking powder and gently stir with a spoon or spatula.
Pour the batter to the tin, put it to the hot oven and bake for 30-40 minutes until an inserted wooden stick comes out dry.
Liquidise the strawberries; press them through a fine sieve to remove the seeds, add 3 tbsp of icing sugar and stir.
Whip the cream with electric mixer until light and fluffy, add the rest of icing sugar (3 tbsp) and stir well.
Mix the gelatin with the hot water in a cup. If it doesn’t dissolve quickly, put the cup into a bowl with boiling water and stir constantly.
Add a few spoons of the liquidized strawberries to the dissolved gelatin, stir it and pour it to the rest of strawberries, stir again and gently combine with the whipped cream.
Take the cold sponge out from the baking tin and cut horizontally in half.
Cut open the plastic folder, so you have one big single-layered rectangle. Then cut in half along the longer side, so you end up with 2 long strips.
Line the sides of the tin with the plastic strips overlapping each other and place the bottom half of the cake back in the tin.
Cut the strawberries in half and start placing them one by one on the edge of the cake so that the cut part touches the plastic lining on the side of the tin. While you do it, press the strawberries gently into the cake, so they stay vertical.
Pour the mousse on top of the cake and spread it making sure the strawberries stay in place (next to the edge of the tin). You can keep a few tablespoons of the mousse and spread it on top.
Place carefully the top part of the cake on the mousse and decorate it with strawberries or the leftover mousse.
Put the cake to a fridge to set for at least 2 hours.
Before serving, take the cake out from the fridge, open and lift the side of the tin and gently remove the plastic strips. This way you can be sure the edges and the pattern will not be damaged.