Bloom the gelatin by sprinkling it over the cold water in a small bowl; let it sit for 5 minutes until softened.

In a blender or food processor, combine the raspberries and granulated sugar, then blend until smooth; press the puree through a fine-mesh sieve to remove seeds.

Gently warm the strained raspberry puree over low heat, then stir in the bloomed gelatin until fully dissolved; remove from heat and let cool to room temperature.

In a separate large bowl, whip the cold heavy cream until soft peaks form, then carefully fold the cooled raspberry mixture into the whipped cream along with the optional lemon juice until just combined.

Divide the mousse into individual serving dishes and chill in the refrigerator for at least 2 hours, or until set.