Add the basil, water, lemon juice, and garlic to a blender or food processor, and process until broken down and relatively smooth.
Add the avocado and salt, and blend again until smooth and creamy. Adjust any seasoning to taste, then serve over your favorite pasta or veggies.
This recipe makes enough sauce for 1 pound of pasta, but leftover sauce can be saved in an airtight container in the fridge for up to 48 hours without much browning. For longer storage, store the leftover sauce in an airtight container in the freezer for up to 3 months.