In a mixing bowl, whisk together the olive oil, lemon juice, minced garlic, salt, and pepper until combined.
Stir in the diced onion to the marinade.
Place the chicken breasts into a gallon-size resealable bag and pour in the marinade, ensuring each piece is coated; seal the bag and refrigerate to marinate overnight (about 8–12 hours).
Preheat the grill to medium-high heat.
Remove chicken from the marinade and place on the preheated grill. Sprinkle with additional pepper if desired.
Grill the chicken for about 20–30 minutes, turning occasionally, until the internal temperature reaches 165°F (75°C) and there is no pink in the center.
Remove from the grill and let rest a few minutes before serving.