Arrange the rack in the middle of the oven, then preheat the fan-assisted oven to 220 °C / 430 °F or the conventional oven to 230 °C / 445 °F. Line a baking sheet with parchment paper.
Season the fish
Pat dry the fish using paper towels and place it on a working surface. Season the interior of the fish with salt and pepper. Halve the lemon slices and add three halves of lemon into each fish cavity along with the fennel or parsley and crushed garlic.
Tie the fish
Using a kitchen string tie the fish in four places. This will prevent the lemon and fennel from falling out. Drizzle with olive oil and season with salt on top.
bake in the oven
Place the fish on a prepared baking sheet and place it in the preheated oven on the middle rack. Bake for 16 - 18 minutes at the fan-assisted oven to 220 °C / 430 °F or the conventional oven at 230 °C / 445 °F.
While the fish is baking, make the sauce. Peel the onion and slice it thinly using a mandolin. If you don’t have a mandolin at home, finely dice the onion. Place a skillet over medium heat. Add a tablespoon of butter and a teaspoon of olive oil. When the butter melts, add the onion and capers. Cook for 2 minutes or until the onion gets translucent, but it shouldn’t brown. Increase the heat and pour in the wine. Stir vigorously and boil the wine for 2 minutes for the alcohol to evaporate. Remove from the heat and stir in the butter and sweet corn. Stir until the butter melts, then set aside until needed.TipOptionally warm the sauce by placing it over low heat.
Remove the fish from the baking sheet, untie and serve with butter sauce and your favorite side.