01 - Bring a large pot of generously salted water to a rolling boil. Add pasta and cook according to package directions until al dente. Before draining, ladle out 1/2 cup of the starchy cooking water and set aside. Drain the pasta thoroughly.
02 - While the pasta cooks, combine avocados, spinach, garlic, lemon juice, olive oil, nutritional yeast or Parmesan, salt, and pepper in a food processor or high-speed blender. Process until completely smooth and velvety, scraping down the sides as needed. Thin with a splash of reserved pasta water if the mixture is too thick.
03 - Return the drained pasta to the warm pot. Pour the avocado-spinach sauce over the noodles and toss gently but thoroughly to coat. Add the reserved pasta water in small increments, tossing continuously, until the sauce achieves a silky consistency that clings to every strand.
04 - Divide the dressed pasta among four shallow bowls. Scatter chopped walnuts and lemon zest over each portion. Tuck fresh basil leaves among the noodles and finish with a light dusting of Parmesan or nutritional yeast if desired.
05 - Present at once while the sauce is at its brightest green and most aromatic. The avocado base will oxidize if left standing, so prompt service preserves both visual appeal and fresh flavor.