01 - Line the base of a 20 cm (8-inch) springform pan with parchment paper and set aside.
02 - Pulse the graham crackers in a food processor until fine crumbs form. Transfer to a bowl, add the melted butter and sugar, and stir until the mixture resembles wet sand and holds together when pressed.
03 - Firmly press the crumb mixture into the base of the prepared springform pan, creating an even layer. Place in the refrigerator to set while you prepare the filling.
04 - If using gelatin sheets, soak them in cold water for 5 minutes until softened. If using powdered gelatin, sprinkle it evenly over 3 tablespoons of cold water and let it bloom for 5 minutes.
05 - Transfer the bloomed gelatin (and its soaking water, if using sheets) to a small saucepan over low heat. Stir gently just until fully dissolved—do not boil. Remove from heat and let 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 lump-free.
07 - In a separate chilled bowl, whip the cold heavy cream with an electric mixer until soft peaks form—do not overwhip.
08 - Gently fold the whipped cream into the cream cheese mixture in two additions, using a spatula and light folding motions to preserve the airy texture.
09 - Stir about 3 tablespoons of the cheesecake filling into the dissolved gelatin to temper it. Then fold this mixture back into the main filling until evenly combined.
10 - Pour the pistachio filling over the chilled crust and spread evenly with a spatula, smoothing the top.
11 - Refrigerate the cheesecake for at least 4 hours, or preferably overnight, until the filling is firm and fully set.
12 - Carefully remove the cheesecake from the springform pan. Garnish with chopped pistachios and fresh berries or mint leaves if desired. Slice into 8 portions and serve chilled.