Place the pistachios in a dry skillet over medium heat. Stir and flip the nuts frequently for 3-4 minutes until the nuts are fragrant and very lightly toasted. Transfer to a plate and let cool fully.

Add the cooled pistachios to a food processor. Add the basil, arugula, garlic, lemon juice, crushed red pepper flakes, and salt. Pulse several times until the mixture is finely chopped and begins to form a coarse paste. Scrape down the sides of the bowl as needed to keep the mixture moving evenly through the blades.

With the machine running, slowly pour in the olive oil in a steady stream so the mixture emulsifies and becomes smooth.

Add the ice and process again until the sauce loosens to a spoonable consistency. Scrape the sides of the bowl once more and process briefly to ensure an even texture. Taste and tweak the seasoning if needed.