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