Rinse the fish thoroughly under cold water.
Season the inside and outside with salt and black pepper.
Optionally, insert a few slices of garlic or a sprig of thyme inside the cavity for added flavor.
Cover and refrigerate while preparing the vegetables.
Preheat the oven to 200°C (392°F).
In a large ovenproof dish, heat the olive oil over medium heat.
Sauté the sliced onion until translucent.
Add the sliced potatoes, season with salt and pepper, and stir to combine.
Pour the diluted tomato paste over the potatoes, adding enough water to cover them halfway.
Cover the dish with a lid or aluminum foil and bake for 30 minutes.
Remove the dish from the oven.
Place the seasoned sea bream on top of the partially cooked potatoes.
Arrange the tomato and lemon slices over the fish.
Scatter olives and fresh herbs over the top.
Cover and return to the oven for an additional 10 minutes.
Uncover and bake for another 10 minutes, or until the fish is cooked through and the top is slightly golden.
Remove from the oven and let rest for a few minutes.
Serve hot, ensuring each plate has a portion of fish and the vegetable medley.