In a small bowl, combine cabbage, onion, lemon juice, salt, and pepper. Mix well and divide mixture onto both foil sheets. Lay cod filets on top, then drizzle the dressing on top of the fish. Take the lower and upper end of the foil and crimp it together going all the way down on both sides to seal the package. Transfer the fish onto a baking sheet and bake for about 15 minutes.