Peel the onion and finely slice with the tomatoes and lemon. Roughly chop the dill and parsley (stalks and all).
Drizzle a large baking tray with oil, scatter over a third of the sliced tomato, lemon and herbs, then top with the fish, seasoning the cavity and skin with sea salt and black pepper.
Stuff the cavity with another third of the sliced tomato, lemon and herbs, then scatter the remaining third over the top – arranging a few tomatoes around the fish. Dot with butter, then drizzle all over with 2 tablespoons of oil and a twist of black pepper.
Roast for 25 minutes, or until just cooked through. Delicious served with samphire.