In a medium saucepan over low heat, combine the fresh raspberries and granulated sugar. Stir gently until the berries soften and release their juices, about 5-7 minutes. If using, add the lemon juice. Let it cool slightly.

Transfer the cooled raspberry syrup to a blender and puree until smooth. Strain through a fine-mesh sieve to remove seeds for a silky texture.

If using gelatin, sprinkle it over cold water in a bowl and let sit for 5 minutes. Heat in the microwave until melted, then mix with cooled raspberry puree.

In a large mixing bowl, pour in chilled heavy whipping cream, powdered sugar, and vanilla extract. Whip on medium-high speed until stiff peaks form, about 3-5 minutes.

Gently fold a portion of the raspberry puree into the whipped cream. Continue adding the raspberry puree in sections until well incorporated while maintaining fluffiness.

Pipe or spoon the mousse mixture into serving glasses, cover with plastic wrap, and refrigerate for at least 2 hours to firm up.

Before serving, garnish with fresh raspberries or a mint sprig for elegance. Serve cold.