Prep a 9-inch springform pan with cooking spray and parchment paper. In a bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Stir until the mixture resembles wet sand.
Press the crust firmly into the bottom of the pan using the back of a spoon or a flat-bottomed glass. Place in the refrigerator while preparing the mousse.
Bloom the gelatin. In a small bowl, sprinkle unflavored gelatin over cold water. Let it sit for 10 minutes until it softens. Then, microwave for 10-15 seconds until fully dissolved. Stir well.
Make the strawberry puree. Blend fresh strawberries until smooth. If desired, strain to remove seeds. Stir the dissolved gelatin into the strawberry puree until fully mixed. Set aside.
In a large bowl, beat cold heavy whipping cream, lemon juice, powdered sugar, and vanilla extract with an electric mixer until stiff peaks form.
Gently fold the strawberry mixture into the whipped cream using a spatula. Be careful not to deflate the mousse.
Pour the mousse mixture over the crust. Spread the mousse evenly over the chilled crust, smoothing the top with an offset spatula. Cover and refrigerate for at least 6 hours or overnight to allow the mousse to firm up.
Carefully run a knife around the edges of the pan before releasing the springform. Garnish with dollops of whipped cream, and fresh strawberry slices on top of each dollop.