Preheat the oven to 375°F.
Peel the potatoes and slice them into 1/8-inch (3–4 mm) rounds.
Thinly slice the onion and cut the green bell pepper into strips.
Drizzle a rimmed baking dish with some of the olive oil. Arrange the potato slices in a single layer, then scatter the onion and bell pepper on top. Season with salt and pepper, and drizzle with the remaining olive oil.
Bake the vegetables for 15–20 minutes, until they begin to soften.
Meanwhile, rinse the sea bream under cold water and pat it dry. Make shallow cuts on each side of the fish and season inside and out with salt and pepper.
Finely chop the garlic and parsley, mix them with a little olive oil, and slice half of the lemon into thin rounds.
Remove the baking dish from the oven and place the sea bream on top of the vegetables. Stuff some lemon slices and a bit of the garlic-parsley mixture into the cavity of the fish, then drizzle the rest over the fish.
Pour the white wine evenly over the fish and vegetables.
Return to the oven and bake for another 20–25 minutes, depending on the size of the fish, until the flesh is opaque and flakes easily with a fork.
Remove from the oven and let the fish rest for a couple of minutes before serving. Serve hot with the roasted vegetables.