Wash, hull, and roughly chop the strawberries.
Blend the strawberries with the sugar and lemon juice until smooth.
Pour the strawberry mixture into a small saucepan and warm gently over low heat.
In a small bowl, sprinkle the gelatin over the cold water and let it sit for about 5 minutes.
Add the softened gelatin to the warm strawberry mixture and stir until completely dissolved.
Remove the strawberry mixture from the heat and allow it to cool to room temperature.
In a large mixing bowl, whip the heavy cream with vanilla extract until soft peaks form.
Gently fold the cooled strawberry mixture into the whipped cream.
Continue folding carefully until the mixture is evenly combined without losing too much air.
Divide the mousse among six dessert glasses or serving cups.
Cover and refrigerate for at least 2 hours, or until set.
Garnish with fresh strawberry slices and whipped cream before serving.
Serve chilled.