01 - Bring a large pot of generously 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 (if using), 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 vibrantly green, scraping down the sides as needed.
03 - Stop and taste the sauce. Adjust salt, pepper, or lemon juice as desired 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 preferred consistency and coats every strand evenly.
05 - Divide the pasta among serving bowls. Finish with an extra sprinkle of Parmesan cheese, freshly chopped herbs, and a light drizzle of olive oil if desired.