Follow these simple steps to create your Strawberry Mousse Cake Filling:

1. Prepare Strawberries: In a medium bowl, combine the sliced strawberries, granulated sugar, and lemon juice. Let them sit for about 10 minutes to macerate, allowing the flavors to meld.
2. Blend Strawberries: After the strawberries have released their juices, use a blender or food processor to puree them until smooth. Set aside.
3. Dissolve Gelatin: In a small bowl, sprinkle the gelatin over cold water. Let it sit for about 5 minutes to bloom.
4. Heat Gelatin: Gently heat the bloomed gelatin in the microwave for about 10-15 seconds until fully dissolved. Allow it to cool slightly.
5. Combine Strawberry Puree and Gelatin: Stir the dissolved gelatin into the strawberry puree until fully incorporated.
6. Whip Cream: In a large mixing bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract until soft peaks form.
7. Fold Mixtures Together: Gently fold the strawberry mixture into the whipped cream. Be careful not to deflate the whipped cream too much. Mix until just combined and the color is uniform.
8. Chill the Mousse: Transfer the mousse to a bowl and cover it with plastic wrap. Refrigerate for at least 1 hour to allow it to set.
9. Use as Filling: Once chilled and set, your Strawberry Mousse Cake Filling is ready to be used in your favorite cake or dessert!