Spray 14-inch-long sheet of heavy-duty aluminum foil with nonstick cooking spray.
Place onion slices in center of foil and top with feta slices.
Sprinkle with bell pepper slices, oregano and garlic pepper.
Seal foil by bringing two long sides of foil together over cheese and peppers; fold down in series of locked folds, allowing for heat circulation and expansion.
Fold short ends up and over again.
Press folds firmly to seal packet.
Place foil packet on grid upside down and grill on covered grill over hot coals 15 minutes.
Turn packet over; grill 15 minutes more.
Open packet; serve immediately with French bread.