In a food processor with the "S" blade OR a high-speed blender, puree the avocado, bananas, half the pistachios, agave or stevia, vanilla, and salt until smooth. This will take a few minutes and will be like soft serve ice cream. Add a little plant-based milk if necessary for blending.
Fold in the other half of the pistachios.
For firmer ice cream, place in freezer for about an hour, stirring it occasionally.