In a small bowl, make marinade by mixing together the onion, 1/3 cup of dressing, garlic, salt and pepper.
Place chicken in a gallon size bag and pour marinade mixture over top.
Let sit in refrigerator for 30 minutes to 1 hour.
Cook tortellini according to package directions. Drain and rinse with cold water.
Cook the chicken until no longer pink, let rest for a couple minutes and then slice.
In a large bowl, combine salad mix, tomatoes, olives, tortellini and chicken.
Drizzle with the remaining dressing; toss to coat.
Sprinkle with cheese.