01 - Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta.
02 - In a food processor or blender, combine avocados, spinach, garlic, Parmesan, lemon juice, olive oil, chili flakes if using, salt, and black pepper. Blend until smooth and creamy. Adjust consistency with reserved pasta water if needed.
03 - Return the drained pasta to the pot. Pour the avocado-spinach sauce over the pasta and toss thoroughly to coat, adding reserved pasta water as necessary for a silky texture.
04 - Plate immediately, garnishing with fresh basil leaves and extra Parmesan cheese.