Prepare the simple syrup by stirring warm water, lemon juice, and granulated sugar until sugar dissolves. Set aside.

In a small bowl, sprinkle gelatin over ¼ cup cold water and let bloom for 5 minutes.

In a saucepan over low heat, warm the bloomed gelatin just until dissolved; do not boil. Remove from heat.

In a blender or food processor, puree frozen raspberries with granulated sugar and lemon juice until smooth. Strain to remove seeds if desired.

Whisk the warm gelatin mixture into the raspberry puree until fully combined. Allow mixture to cool to room temperature.

In a chilled bowl, whip heavy cream with powdered sugar to stiff peaks.

Gently fold the raspberry mixture into the whipped cream until evenly blended.

Dip lady fingers briefly into simple syrup and layer in a serving dish or individual glasses.

Spoon half of the mousse over the lady fingers, then add a layer of fresh raspberries. Repeat layers ending with mousse on top.

Chill in the refrigerator for at least 4 hours or until set.