Combine the graham cracker crumbs, melted butter, and sugar in a mixing bowl. Stir until it resembles wet sand, then press into the bottom of a springform pan. Chill for about 30 minutes.

Beat the heavy cream until soft peaks form, about 3-4 minutes. In another bowl, blend mascarpone, powdered sugar, vanilla extract, and lemon juice until smooth. Fold the whipped cream into the mascarpone mixture.

Wash and dry the mixed berries, then fold half into the filling, being careful not to break them. Reserve the remaining berries for garnish.

Pour the filling into the crust, spreading evenly. Smooth the top and cover with plastic wrap. Refrigerate for at least 4 hours or overnight.

Remove from the refrigerator, release from the pan, and top with reserved berries. Slice and serve.