Pre-heat the oven to 180 degrees. Arrange the onion, potato and most of the tomato in the base of a ceramic roasting dish large enough to hold the bream. Drizzle half of the olive oil on top and then half of the lemon juice and some seasoning.
Sit the bream on top of the vegetables and arrange few lemon slices on top either side of a slice of tomato. Drizzle the remaining olive oil and lemon juice on top. Season again and sprinkle the chopped parsley evenly on top.
Cover the dish with aluminium foil and bake for 25-30 minutes, then remove the foil and cook for a further 10 minutes uncovered. Remove and allow the fish to sit for a few minutes before serving directly from the dish.
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