Prepare the Strawberries: In a blender or food processor, combine the sliced strawberries, granulated sugar, and lemon juice. Blend until smooth. Taste and add more sugar if needed, depending on the sweetness of your strawberries. You should have a smooth strawberry puree.

Strain the Puree (Optional): For an extra smooth mousse, pour the strawberry puree through a fine-mesh sieve to remove the seeds. This step is optional, but it will give you a silkier texture.

Whip the Heavy Cream: In a large, chilled bowl, pour in the cold heavy cream, powdered sugar, and vanilla extract. Use an electric mixer (hand mixer or stand mixer) to whip the cream until stiff peaks form. This means when you lift the whisk, the cream holds its shape and doesn't droop. Be careful not to overwhip, or you'll get butter!

Combine and Fold: Gently fold the strawberry puree into the whipped cream in two or three additions. Use a spatula and make sure to fold gently to keep the mousse light and airy. Fold until just combined and you see no more streaks of white cream or pink puree.

Chill: Divide the Strawberry Mousse Dessert into individual serving glasses or bowls. Cover them with plastic wrap and refrigerate for at least 2 hours, or preferably 4 hours, to allow the mousse to set and become firm.