In the bowl of a food processor or a high-speed blender, add the diced zucchini, basil, garlic, walnuts, lemon juice, salt, and 3 tablespoons of water. Blend until the pesto looks uniform, adding an extra tablespoon of water, if needed to help everything blend smoothly. Stop and scrape down the sides of the machine, if needed.
For a more authentic pesto flavor, add in the optional Parmesan cheese and olive oil, then blend again. Taste the pesto, and season with extra salt (I add in an extra 1/4 teaspoon in this case) or a pinch of black pepper, if desired.
Once the pesto is to your liking, serve it right away over your favorite pasta, pizza, sandwiches, and more. Leftover pesto can be stored in an airtight container in the refrigerator for up to 3 days.
