Chill the cream: Make sure the heavy cream is thoroughly chilled. Keep it in the refrigerator for at least 4 hours before whipping.
Whip the cream: Pour the cold heavy cream into a mixing bowl. Starting at medium speed and gradually increasing to high, whip until firm peaks form. Cover and refrigerate immediately.
Prepare the strawberries: Puree the strawberries in a food processor or with an immersion blender until smooth.
Cook the strawberry puree: Transfer the strawberry puree to a saucepan. Add the powdered sugar and lemon juice. Cook over medium-low heat, stirring frequently, until the mixture reduces to approximately ⅔ cup.
Cool completely: Remove from the heat and stir in the strawberry extract, if using. Set aside and allow the strawberry mixture to cool completely to room temperature.
Combine: Gently fold the cooled strawberry mixture into the whipped cream until evenly incorporated and no streaks remain. Be careful not to deflate the whipped cream.
Chill: Divide the mousse among individual ramekins or serving glasses. Refrigerate for at least 4 hours, or until set.
Garnish and serve: Just before serving, garnish with a little strawberry jam and fresh strawberries. Serve chilled and enjoy!