Bring a large pot of salted water to a boil over high heat. Add the pasta, and cook according to package instructions (al dente). Drain and rinse the pasta in cold water, allowing it to cool completely.

Meanwhile, blend the mayonnaise, yogurt, parsley, basil, lemon juice, lemon zest, anchovy paste, salt, and pepper in a small blender, food processor, or with an immersion blender.

Toss the cooled pasta, onion, and 3/4 of the dressing together in a large salad bowl. Just before serving, toss in the spinach, arugula, and chives and serve in shallow salad bowls or plates topped with sliced avocado.

Drizzle the remaining dressing over the top and sprinkle with salt, freshly cracked black pepper, fresh basil, and freshly grated parmesan cheese for garnish if desired.