01 - Bring a large pot of salted water to a boil. Cook pasta until al dente, then reserve 1/2 cup of cooking water before draining.
02 - Combine avocados, spinach, garlic, olive oil, milk, Parmesan, lemon juice, salt, pepper, and red pepper flakes in a blender or food processor.
03 - Blend until completely smooth and creamy. If sauce is too thick, add small amounts of reserved pasta water to reach desired consistency.
04 - Return drained pasta to the pot. Pour sauce over pasta and toss gently to coat evenly, adding more reserved pasta water as needed for a silky texture.
05 - Serve immediately topped with extra grated Parmesan, fresh basil leaves, and toasted pine nuts if desired.