Preheat the oven to 190°C/Gas Mark 5 (conventional heat, not fan).
Peel the potatoes and slice them very thinly (about 3–4 mm thick).
Cut the onion into thin strips and slice the green pepper into strips.
Drizzle a little extra-virgin olive oil over a baking tray. Arrange the potatoes in a single layer, scatter over the onion and pepper, season with salt and freshly ground black pepper, then drizzle over a little more extra-virgin olive oil.
Bake the vegetables for 15–20 minutes, or until they begin to soften.
Meanwhile, if the sea bream is not already scaled and gutted, rinse it under cold water and pat dry. Make two or three shallow slashes in the flesh on each side and season inside and out with salt and freshly ground black pepper.
Peel and finely chop the garlic cloves and chop the parsley. Stir them into a little extra-virgin olive oil. Cut the lemon into thin slices.
Remove the tray from the oven and place the sea bream on top of the softened vegetables. Slip some of the lemon slices and part of the garlic and parsley mixture into the cavity of the fish, then spread the rest over the top.
Pour the dry white wine over everything.
Return the tray to the oven and bake for a further 20–25 minutes (depending on the size of the fish), until the flesh is opaque and flakes easily with a fork.
Remove the tray from the oven and leave the fish to rest for a couple of minutes.
Serve the sea bream hot with the roasted vegetables.