Fold a double layer of aluminum foil into a square. Place 1 tilapia fillet into the middle of the square. Repeat the process for each fillet.
In a small bowl, whisk together olive oil, lemon juice and seasonings. Place pepper and onion slices on top of each fillet, then drizzle with olive oil blend, about 2 teaspoons per fillet.
Fold up the edges and close each packet. Freeze or cook immediately. To cook right away, place packets on a cookie sheet and cook at 350 degrees for 10-15 minutes, until the fish is fully cooked. (Increase cooking time if using frozen fish fillets.) Or, place packets on a grill and cook until done.
If you freeze the packets right after putting them together, cook as above straight from the freezer and increase the cooking time about 10 minutes.