Consider this a savory crumble, with fish in the filling and cornbread in the topping. The fish is simply seasoned to taste with salt, pepper, and fresh lemon juice before being layered atop onion slices in the baking dish, with the topping sprinkled evenly over the fish. A drizzle of oil is all the fat that's needed to carry the flavors and to keep the fish moist as it cooks. Serve hot, with more lemon slices for squeezing.