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, place the basil, parsley, chives, tarragon (if using), avocado, Greek yogurt, olive oil, lemon juice, garlic, and Parmesan into a food processor or blender. Process on high until the mixture is completely smooth and yields 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, blend in a few tablespoons of the reserved pasta water, one tablespoon at a time, until it reaches a pourable, silky consistency.
04 - Transfer the drained pasta to a large mixing bowl. Pour the green goddess sauce over the pasta and toss vigorously until every strand or piece is evenly coated. Add additional pasta water as needed to achieve a luscious, creamy texture that clings to the pasta.
05 - Plate immediately while warm. Garnish with additional fresh herbs and an extra sprinkle of Parmesan if desired. Serve alongside a crisp white wine such as Sauvignon Blanc or Pinot Grigio for a complete meal.