Heat oven to 350 degrees. Place 12-inch piece of foil on counter. Coat foil with nonstick cooking spray.
Place fillets in middle of foil. If fillets have skin, place skin-side down. Spread sliced onions, salt, pepper and oil on top of fillets. Add lemon pepper seasoning, if desired. Fold foil over fish.
Place foil pouch on baking sheet and place in oven. Bake fish for 15-20 minutes until fish reaches a minimum internal temperature of 145 degrees on a food thermometer and is flaky when tested with fork.
Divide into four portions and serve.