01 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package instructions until al dente. Before draining, reserve 1/2 cup of the starchy pasta water. Drain the pasta through a colander and set aside.
02 - While the pasta cooks, add the avocado, spinach, basil, chives, parsley, tarragon, garlic, olive oil, Greek yogurt, lemon juice, Parmesan cheese, salt, pepper, and chili flakes (if using) to a food processor or blender. Blend on high until completely smooth and the mixture turns a vibrant, uniform green color.
03 - Taste the sauce and adjust the salt, pepper, or lemon juice as needed to balance the flavors.
04 - Return the drained pasta to the pot or transfer to a large mixing bowl. Pour the green goddess sauce over the hot pasta and toss thoroughly, adding splashes of the reserved pasta water until the sauce reaches your desired creamy consistency.
05 - Divide the sauced pasta among serving bowls. Garnish with extra Parmesan, freshly chopped herbs, and a light drizzle of olive oil if desired. Serve immediately.