Add the arugula, spinach, basil, garlic, walnuts, salt, and parmesan to a food processor. Pulse the ingredients together until well broken down. You may add more cheese and garlic if desired.
Keep the motor of the food processor running and add the oil a little at a time. Continue blending and adding until you have a semi-coarse sauce. You may add more or less oil than recommended as desired to reach your preferred consistency.
Taste the pesto and adjust seasonings and flavorings to taste. You can add more basil or salt and more oil if you’d prefer a thinner consistency. Store in the fridge in an airtight container until ready to use.