Wash the fish and pat dry. Score both sides of the fish. Massage it with 1/2 tablespoon of olive oil and sprinkle both the outside and the cavity of the fish generously with salt and pepper.
Cut lemon and onion into thin slices. Stuff lemon slices, onion slices, and dill into the cavity of the fish.
When the oven is ready, place the fish on a baking pan lined with parchment paper.
Use a spatula to slide under the fish and together with a pair of tongs, flip it. Cook for another 5-8 minutes (amount of time depends on how big your fish is).
When the fish is fully cooked, transfer to a serving plate lined with fresh dill and thin sliced lemon. Drizzle lemon caper sauce over it and enjoy.
For the lemon caper sauce:
While the fish is cooking, put ingredients to make the sauce into a bowl and mix well to combine.