Peel and chop the mangoes into small pieces, then puree them in a blender until smooth.

In a small bowl, combine the gelatin and water. Allow it to sit for about 5 minutes until it blooms, then heat in the microwave for approximately 10-15 seconds until fully dissolved.

In a large bowl, mix the pureed mango with sugar and lemon juice. Stir until the sugar is dissolved.

Gently fold in the dissolved gelatin mixture.

In another bowl, whip the heavy cream until soft peaks form, being careful not to over-whip.

Gradually fold the whipped cream into the mango mixture until smooth and even.

Place the sponge cake or graham cracker crust in a springform pan and pour the mango mousse over the top. Smooth the surface with a spatula.

Cover the cake with plastic wrap and refrigerate for at least 4-6 hours, or until the mousse has set.

Once set, carefully remove from the pan and slice into portions. Garnish with fresh fruit or mint if desired.