1. Make a slurry by mixing the 1/4 cup of the milk with the cornstarch, mixing until the starch is dissolved and the mixture is smooth.
2. Heat the rest of the milk in a medium-sized saucepan with the sugar.
3. When it almost starts to boil, stir in the cornstarch mixture and cook at gentle simmer for 3 minutes, stirring constantly.
4. Remove from heat, scrape into a bowl, and chill thoroughly, preferably overnight.
5. Once chilled, whisk in the pistachio paste and just a few drops of citrus juice until smooth.
6. Freeze the gelato in your ice cream machine according to the manufacturer’s instructions.