Place the strawberries, granulated sugar, and lemon juice in a blender and blend until smooth. Strain the mixture through a fine sieve to remove seeds, if desired.

In a small saucepan, combine the cold water and agar-agar powder. Heat over medium-low heat, stirring continuously, until the agar-agar dissolves completely. Remove from heat and let cool slightly.

Mix the agar-agar solution into the strawberry puree and stir well to combine. Set aside to cool to room temperature.

In a large mixing bowl, whip the chilled heavy cream, powdered sugar, and vanilla extract until soft peaks form.

Gently fold the cooled strawberry puree into the whipped cream, using a spatula to combine until smooth and fluffy.

Spoon the mousse into serving glasses or bowls, smoothing the tops. Cover and refrigerate for at least 2 hours to set.

Garnish with fresh strawberries or mint leaves before serving for an extra touch of elegance.