To the bowl of a food processor or Vitamix blender add basil, garlic, walnuts, parmesan, lemon zest, juice, and red pepper flakes.
Pulse a few times to break down the basil.
In a steady stream add olive oil, pulsing until olive oil is blended into other ingredients and mixture forms a loose paste. If your pesto is too thick, add additional olive oil one tablespoon at a time until you reach the desired consistency.
Season with salt and pepper.
Enjoy!
Video
Notes
Store the pesto in a jar with a tight-fitting lid in the refrigerator for up to 5 days. Spread the top of the pesto into an even layer and top it with olive oil to cover it. This will prevent the pesto from oxidizing and turning dark.