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