Line pan sides with ladyfingers vertically.
Make raspberry purée in a saucepan over heat.
Add lemon juice and sugar, stir until dissolved.
Strain mixture to remove seeds, cool slightly.
Dissolve gelatin in cold water, then warm gently.
Stir gelatin into purée, mix until fully combined.
Whip cream until stiff peaks form.
Fold whipped cream into raspberry mixture gently.
Pour mousse into prepared pan, spread evenly.
Chill cake for at least six hours or overnight.
Garnish with raspberries, mint, or powdered sugar.