Combine marinade mix with water, oil and vinegar in glass dish or large self-closing plastic bag.
Add chicken, turning to coat all sides. Marinate in refrigerator 30 minutes, or longer for extra flavor.
Preheat grill or grill pan to medium. Cook chicken 4-5 minutes per side or until done, basting with remaining marinade halfway through cooking, if desired. Discard any leftover marinade.
Rub or brush red onion slices with olive oil; season with salt and pepper. Grill over medium heat 3-4 minutes per side.
In a medium bowl, combine salsa ingredients.
Serve chicken atop grilled red onion slices; top with salsa.