Put ¼ of the avocado, 1 cup basil, 1 garlic clove, 1 teaspoon lemon juice, and 1 tablespoon nutritional yeast in a food processor or blender and blend. Add the water 1 tablespoon at a time until you get the desired consistency. Add salt and pepper to taste.
Enjoy!
Notes
To preserve the remaining portion of the avocado - squeeze some lemon juice on the rest of the avocado and seal it up tight with plastic wrap. It will stay fresh for the next day and you can use it for guacamole, or a sandwich, or a salad, or just eat it as is with a little balsamic vinegar drizzled on top.
If it turns slightly brown overnight, just slice off the top. No big deal. You could also just mash the avocado, mix in a bit of lime juice, put it in an airtight container, put a layer of plastic wrap on the avocado and then put a bit of water on the plastic to prevent air from oxidizing it, cover it and keep it in the fridge for up to three days.
To make a large batch of pesto - use the entire avocado, add and extra two to three teaspoons (taste and add to get the right amount) of nutritional yeast, two to three (add and taste so it’s not overpowering) garlic cloves, three more cups of basil leaves, extra salt and pepper to taste, and as much water as it takes to whip it into the perfect texture.
Substitution - If you don't like nutritional yeast, feel free to substitute with grated vegan parmesan cheese.