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 basil, parsley, chives, tarragon (if using), avocado, Greek yogurt, olive oil, lemon juice, garlic, and Parmesan cheese into a food processor or blender. Blend on high until the mixture is completely smooth and turns a vibrant green color, scraping down the sides as needed.
03 - Taste the sauce and season with salt, pepper, and chili flakes if desired. If the sauce is too thick, gradually blend in small amounts of the reserved pasta water until you reach a creamy, pourable consistency.
04 - Transfer the drained pasta to a large mixing bowl. Pour the green goddess sauce over the pasta and toss thoroughly until every strand or piece is evenly coated. Add additional pasta water as needed to achieve a silky, glossy finish.
05 - Divide among plates or bowls immediately. Garnish with extra fresh herbs and an additional sprinkle of Parmesan cheese if desired. Serve warm.