Preheat oven to 400 degrees
Rub salt and pepper into both sides of the fish and place the cod on a baking sheet. (I line the baking sheet with parchment paper for easier clean up but it is not necessary)
Cut the lemon and onion either in rings or semi-circles and chop the pepper.
Place the lemon, onion, pepper, and butter on the fish.
Optional: Squeeze some fresh lemon juice on top of the fish in addition to the lemon.
Place the fish into the oven for 10-15 minutes. (I put mine in for 12)
Check to make sure the fish is fully cooked with a fork. It should fall apart and flake easily. Enjoy!