01 - Bring a large pot of salted water to a rolling boil. Cook spaghetti or linguine according to package directions until al dente. Reserve 1/2 cup of the pasta cooking water, then drain using a colander.
02 - While the pasta cooks, blend avocados, garlic, fresh basil, lemon juice, olive oil, water, salt, and black pepper in a blender or food processor. Process until the mixture is completely smooth and creamy. Taste and adjust seasoning as needed.
03 - Return drained pasta to the pot. Pour avocado sauce over the noodles and toss thoroughly to achieve even coating. If necessary, add reserved pasta water in small increments until the desired sauce consistency is reached.
04 - Transfer to serving bowls. Top with grated Parmesan cheese if desired, garnish with fresh basil leaves, and add a sprinkle of crushed red pepper flakes for heat.