• Place 1½ ounces Parmigiano Reggiano in the bowl of a food processor and pulse until it's finely grated and resembles coarse sand.
• Add 4 cups packed basil leaves and 1 tablespoon garlic , roughly chopped to the parmesan and pulse until the basil and garlic are finely minced.
• Add 1 teaspoon lemon zest, 1 teaspoon lemon zest, 1 tablespoon lemon juice, ¼ teaspoon red pepper flakes, and a pinch black pepper and pulse to combine.
• Drizzle ½ cup extra virgin olive oil into the basil mixture while processing, until the mixture comes together as a thick sauce. For thinner pistou, drizzle in 2-4 more tablespoons of olive oil to your tastes.

FOR A MORTAR AND PESTLE:

• Use a large mortar and pestle and start with some grated parmesan cheese to add texture and help the mortar break down the garlic and basil. Add a small handful of basil at a time and work it with the pestle to grind and blend until it's all very finely ground.
• Add the remaining ingredients except for the oil and continue to blend until it's a cohesive mixture.
• Slowly blend in the oil until it's incorporated and emulsified. Transfer to a jar.