Marinate chicken breasts in Italian dressing for 1 hour. Slightly pound breasts to keep uniform is shape, trim excess.
Grill both sides 6 to 8 minutes until internal temperature reaches 165 °F, set aside.
Sauté onions in oil until translucent and begin to brown slightly, set aside. Gently sauté tomatoes in oil until warm, sprinkle in salt and pepper.