01 - Line the base of an 8-inch springform pan with parchment paper.
02 - Crush the graham crackers into fine crumbs using a food processor or by placing them in a zip-top bag and rolling with a rolling pin. Combine the crumbs with the melted butter and sugar until evenly moistened.
03 - Press the crumb mixture firmly and evenly into the base of the prepared springform pan. Refrigerate while preparing the filling.
04 - If using gelatin sheets, soak them in cold water for 5 minutes until pliable. If using powdered gelatin, sprinkle it over 3 tablespoons of cold water and let it bloom for 5 minutes.
05 - In a small saucepan, gently warm the soaked gelatin with its liquid just until fully dissolved. Do not boil. Set aside to cool slightly.
06 - In a large mixing bowl, beat the softened cream cheese, pistachio paste, powdered sugar, vanilla extract, and lemon juice with an electric mixer until completely smooth and creamy.
07 - In a separate chilled bowl, whip the cold heavy cream with an electric mixer until soft peaks form.
08 - Gently fold the whipped cream into the cream cheese mixture in two additions, preserving as much volume as possible.
09 - Stir a few tablespoons of the cheesecake filling into the dissolved gelatin to temper it, then fold the gelatin mixture back into the main filling until fully combined.
10 - Pour the filling over the chilled crust and smooth the top with a spatula. Tap the pan gently on the counter to release any air bubbles.
11 - Refrigerate the cheesecake for at least 4 hours, or ideally overnight, until the filling is completely firm and set.
12 - Remove the cheesecake from the springform pan. Garnish with chopped pistachios and fresh berries or mint leaves if desired. Slice with a warm, clean knife and serve chilled.