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 pasta cooking water, then drain the pasta.
02 - While the pasta cooks, place avocados, spinach, garlic, olive oil, milk, Parmesan, lemon juice, salt, black pepper, and red pepper flakes (if using) in a blender or food processor.
03 - Blend until smooth and creamy. If the sauce is too thick, add a little reserved pasta water to reach desired consistency.
04 - Return the drained pasta to the pot. Pour the avocado-spinach sauce over the pasta and toss gently to coat, adding more reserved pasta water as needed to achieve a silky texture.
05 - Serve immediately, garnished with extra Parmesan, fresh basil, and toasted pine nuts if desired.