Sterilize blender and pour thawed raspberries into bowl. Use fork to crush berries slightly, reserving some whole pieces for texture. Add sugar and lemon juice.
Sprinkle gelatin over 2 tbsp cold water to bloom for 5 minutes.
Mix gelatin solution by heating milk in microwave or saucepan until warm, then stir in bloomed gelatin until dissolved.
Blend raspberries, sugar, and lemon juice until smooth. Strain to remove seeds if desired.
Whip heavy cream until loose peaks form. Gently fold into raspberry mixture.
Chill mousse in airtight container for 6 hours or overnight before serving.