01 - Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions 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, place the avocado, spinach, basil, chives, parsley, tarragon (if using), garlic, olive oil, Greek yogurt, lemon juice, Parmesan cheese, salt, pepper, and chili flakes into a food processor or blender. Blend on high until completely smooth and the mixture turns a vibrant, uniform green.
03 - Stop the blender and taste the sauce. Adjust salt, pepper, or lemon juice as needed to balance the flavors. Blend again briefly if any additions are made.
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 to coat every strand. Add reserved pasta water a tablespoon at a time until the sauce reaches your preferred consistency.
05 - Divide the sauced pasta among serving bowls. Finish with an extra sprinkle of Parmesan, a scattering of freshly chopped herbs, and a light drizzle of olive oil if desired. Serve immediately while warm.