Place the raspberries, sugar and lemon juice in a blender or food processor and blend until smooth. Strain the raspberry purée into a bowl, and discard the seeds. Set aside.
Sprinkle the gelatin over water in a small saucepan. Heat the water over low heat, gently stirring until the gelatin dissolves completely, about 1-2 minutes. Stir the gelatin into the raspberry purée, then place in the refrigerator to cool completely and thicken. This will take between 45 minutes to 1 hour.
Place the raspberry gelatin mixture in the bowl of an electric mixer with the whip attachment. Whip the gelatin for about 2 minutes on high speed, until fluffy. While continuing to whip, slowly drizzle the heavy cream into the mixing bowl, then whip for another 2-3 minutes, or until the mixture is combined and thick. Pour into serving glasses and chill for at least 1 hour before serving. If making in advance, cover with plastic wrap before chilling.
Garnish with white chocolate and raspberries before serving.
Notes
Recipe lightly adapted from Raspberry Mousse, Taste of Home.