Rinse the quinoa under cold water, then cook it according to package directions until tender; set aside to cool slightly.
While the quinoa cooks, chop the red onion, halve the tomatoes, slice the cucumber, chop the red bell pepper, slice the avocado, and chop the parsley if using.
Shred or chop the cooked chicken into bite-sized pieces.
In a large mixing bowl, combine the cooled quinoa, cooked chicken, spinach, tomatoes, cucumber, red bell pepper, red onion, and parsley.
Toss the salad with ranch dressing to taste, then season with salt and pepper as needed.
Gently fold in the sliced avocado just before serving to prevent browning.