• Line the bottom and sides of a 9-inch springform pan with parchment paper.
• Cut the sponge cake so that it is 1 inch smaller than the springform pan.
• Trim the ends of the lady fingers so that they will stand upright.

For the simple syrup:

• In a saucepan, heat 1/2 cup of water with the sugar and 2 Tbsp lemon juice, stirring until the sugar has dissolved. Remove from heat and let cool.

For the rhubarb mousse:

• Over low heat in a medium saucepan, heat the rhubarb with 1/2 cup sugar and 1 Tbsp lemon juice. Cook for 15 minutes or until rhubarb has softened. Remove from heat. Measure out 1/2 cup of cooked rhubarb and reserve. Scoop out 1/4 cup of the syrup from the bottom of the pot and reserve. If there isn't enough syrup, top up with water to make 1/4 cup.
• In a saucepan, sprinkle the gelatin over the 1/4 cup rhubarb syrup and let bloom for 5 minutes. Add 1/2 cup of cooked rhubarb and cook, stirring constantly, until the gelatin has dissolved. Let cool and stir in the remaining 3/4 cups cooked rhubarb.
• In a large bowl, whip the cream with the remaining sugar and the vanilla until stiff peaks form. Reserve 1 1/2 cups and set aside. Gently fold in the rhubarb mixture into the remaining cream, using a spatula. Set aside.

To assemble:

• Quickly dip the large ladyfinger pieces in the syrup and place them upright, flat side inward, along the inner walls of the pan.
• Brush the remaining simple syrup over the sponge cake base. Spread the reserved 1/2 cup of cooked rhubarb over the cake base.
• Fill with the rhubarb mousse. Pipe the reserved whipped cream over the mousse and garnish with the strawberries, pressing them lightly into cream. Gently cover the pan with plastic wrap and refrigerate for 8 hours or overnight.