Dice the zucchini if you haven't yet (no need to peel it!) and measure out 1 heaping cup. Or, use a food scale to measure out 5 ounces.
Add the zucchini, basil, garlic, walnuts, lemon juice, salt, and 3 tablespoons of water to a blender. Blend until very smooth, about 60 seconds in a high-speed blender. Make sure the ingredients are totally pulverized and smooth for the best flavor.
Adjust any seasoning to taste, adding more lemon juice or salt, as needed. You can also add 1 more tablespoon of water, if you want a runnier texture. Serve right away, or store the pesto in an airtight container in the fridge for up to 5 days.
Nutrition information is for 1/6 of this recipe, or roughly 1/4 cup. This information is automatically calculated, and is just an estimate, not a guarantee.