Combine graham cracker crumbs, sugar, and melted butter until it resembles wet sand. Press into the bottom and sides of a 9-inch pie dish and chill.
In a saucepan, cook chopped strawberries with sugar, water, and lemon juice for 5–7 minutes until soft and juicy. Blend until smooth and let cool slightly.
Bloom gelatin in cold water for 5 minutes, then gently heat until dissolved. Stir it into the cooled strawberry puree and let it reach room temperature.
Whip heavy cream in a chilled bowl until stiff peaks form. Gently fold in the cooled strawberry mixture, keeping the cream airy.
Spoon the mousse into the crust, smooth the top, and refrigerate for at least 4 hours until set. Garnish with fresh strawberries and whipped cream before serving.