Cook spinach as directed on box. Rinse with cold water until cooled. Drain, squeezing out as much liquid as possible.
In food processor, place spinach and remaining Dressing ingredients. (Do not use blender.) Cover; process 1 to 2 minutes, stopping occasionally to scrape sides, until smooth.
In very large (5-quart) glass bowl, place about half of the salad greens. Layer with chicken, tomatoes and cucumber. Reserve 1/4 cup onion for topping; layer remaining onion over the cucumber. Top with remaining greens.
Spread dressing evenly over salad. Sprinkle with cheese, olives and reserved onion.
Serve immediately, or cover and refrigerate several hours or overnight. Just before serving, toss salad to coat with dressing.