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, add the avocado, spinach, basil, chives, parsley, tarragon (if using), garlic, olive oil, Greek yogurt, lemon juice, Parmesan cheese, salt, pepper, and chili flakes to a food processor or high-speed blender. Process until completely smooth and vividly green, scraping down the sides as needed.
03 - Taste the sauce and adjust salt, pepper, or lemon juice to your preference. The sauce should be bright, creamy, and well-balanced.
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 consistency and coats every strand evenly.
05 - Divide among serving bowls. Finish with an extra shower of Parmesan, a scattering of chopped fresh herbs, and a light drizzle of olive oil if desired. Serve immediately.