Mix the ricotta and lemon zest thoroughly in a stainless steel bowl. Beat the cream and icing sugar together until stiff, mix half into the ricotta using a whisk, then fold in the rest using a rubber spatula. Cover and freeze the mixture for approx. 5 hrs., stirring 3 times.
Mix the melon with the amaretto and lemon balm, allow to infuse briefly. Plate up the ice cream and melon salad, sprinkle with pistachios and garnish with lemon balm as desired.