Keep aside 2-3 strawberries for the final garnish and add the rest in a blender or food processor, puree the fresh strawberries with lemon juice, salt and sugar until smooth.
If using gelatin, soak it over 4 tablespoons of cold water in a small bowl. Let it sit for about 5 minutes to bloom. Then, microwave it for 10-15 seconds until dissolved. Be careful not to overheat. Set aside to cool slightly.
Transfer the strawberry puree to a bowl and reserve ¼ cup of the puree aside for assembling the mousse.
Add the cooled gelatin to the rest of the puree and mix well.
In a large mixing bowl, beat the chilled heavy cream and powdered sugar until stiff peaks form.
Gradually fold the strawberry puree into the whipped cream until well combined. Be gentle to maintain the mousse's airy texture. Use a spatula and avoid stirring vigorously to prevent deflating the whipped cream.
Take a clear glass or serving dish of your choice.
Spoon a layer of the strawberry puree (reserved) into the bottom of the glass.
Add a layer of strawberry mousse on top. Repeat the layers, alternating between the strawberry mousse and strawberry puree until the glass is almost full, leaving a little space at the top.
Cover and refrigerate for at least 4 hours, or until set.
Once set, garnish with additional sliced strawberries on the top. Serve chilled and enjoy!