Add the olive oil, soy sauce, balsamic vinegar, minced garlic and the Good Seasons Italian Dressing package to a 1 gallon Ziploc bag and carefully seal it then swish everything together well. Add chicken breasts and onion. Press out as much of the air from the bag as possible and reseal it. Place on a rimmed dish or plate in case bag leaks. Refrigerate until ready to grill. Flip bag halfway through marinating time.
When you're ready to grill, prepare grill by heating and lightly oiling. Remove chicken and onion from marinade and shake off excess marinade. Discard marinade. Grill chicken on high for about 4-6 minutes, depending on the temperature of your grill. Flip and repeat until done. Grill onions with a foil pan over them, flipping as they brown, for about 15-20 minutes.
When chicken is cooked, remove to a platter to allow the chicken to rest for about 7 minutes. Serve