Special equipment: Ice cream machine
Place the mango chunks, banana, and mango juice into a food processor and process until smooth. Add the frozen yogurt and lemon juice and continue to process until well combined. Transfer the mixture into the frozen insert of a commercial ice cream maker and churn according to manufacturer's instructions. Alternately, put the mixture into a glass baking dish, cover and freeze for 2 to 3 hours, or until frozen. Serve the ice cream in a bowl garnished with diced mango, if using, and pistachios, if using.