Make the dressing: toss the tomatoes and the mozzarella balls into dressing so they're nicely coated (you'll need a tablespoon for each)
Fill a salad bowl with spinach and chopped basil, toss with a tablespoon of dressing to coat everything well.
Top with tomatoes, mozzarella, grilled chicken, and onion.
Drizzle salad with a little more dressing if needed.