Put 1½ cups pistachios in a bowl and cover completely with water, soak for 1 – 8 hours. Drain and rinse pistachios in a strainer under cold running water until the water runs clear.
Chop the remaining ½ cup pistachios with the salt in a food processor until it resembles coarse crumbs. Set aside.
Put the soaked pistachios in a blender with 2¼ cups of water. Process for 2 – 3 minutes or until the mixture is very smooth. It should be the consistency of very heavy cream. If too thick, add a little more water and blend.
Take ½ cup of the pistachio cream and mix with the cornstarch until completely blended. Pour the rest of the pistachio cream into a large saucepan, combine with the chopped pistachios and sugar and cook over medium-high heat until it just starts to boil. Add in the cornstarch mixture and cook for 1 minute, stirring constantly. The mixture should be the consistency of pudding. Take off the heat and stir in the lemon juice. If you want the color a little greener, stir in some green food coloring drops, a few drops at a time, until if becomes the desired shade of green.
For a smooth gelato, push the mixture through a strainer into a bowl or for a chunkier texture just scrape the mixture into a bowl. Top with a piece of plastic wrap placed directly onto the surface to prevent a skin from forming. Let the mixture cool to room temperature then refrigerate for about 2 hours or until cold.
Put mixture into an ice cream machine and process for 20 – 25 minutes or according to the manufacturers instructions.
Can be eaten immediately or stored in the freezer in an airtight container. Take out of freezer for about 5 minutes to soften before serving.
Note – if you make this recipe with salted pistachios, leave the additional salt out of the recipe.