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 ½ cup of the starchy pasta water. Drain the pasta through a colander and set aside.
02 - While the pasta cooks, add the basil, parsley, chives, tarragon (if using), avocado, Greek yogurt, olive oil, lemon juice, garlic, and Parmesan cheese to a food processor or high-powered blender. Blend on high until the mixture is completely smooth and a vibrant green color.
03 - Taste the sauce and season with salt, pepper, and chili flakes as desired. If the sauce is too thick, stream in a few tablespoons of the reserved pasta water while blending until you reach a creamy, pourable consistency.
04 - Transfer the drained pasta to a large mixing bowl. Pour the green goddess sauce over the top and toss thoroughly until every strand or piece is evenly coated. Add additional pasta water as needed to achieve a silky, luscious texture.
05 - Serve immediately in warmed bowls. Garnish with extra fresh herbs, an additional sprinkle of Parmesan, and a drizzle of olive oil if desired.