Preheat your oven to 425 degrees F. You want a fully hot oven before the peppers go in, so give it at least 15 minutes to come up to temperature.

Add the sliced bell peppers and onion to a large mixing bowl. Drizzle the olive oil over the top and sprinkle in the oregano, salt, and black pepper. Toss everything together until the vegetables are evenly coated with oil and seasoning. Take a moment to separate any onion strips that are clumped together.

Spread the pepper and onion mixture out in a single layer on a large rimmed baking sheet. Crowding the pan will steam the vegetables instead of roasting them, which gives you soft and watery rather than caramelized and sweet. If your baking sheet seems too full, use two pans.

Roast for 20 minutes without disturbing the pan. Then use a wide spatula or tongs to flip and toss the peppers. Some edges will look dark and charred at this point. That is exactly what you want. Return the pan to the oven for another 10 minutes.

Pull the pan from the oven. The peppers should be tender, slightly collapsed, and have dark charred spots on some edges. The onions should look golden and soft. Taste for salt and adjust.

If using the red wine vinegar, drizzle it over the hot peppers and toss gently. It brightens the whole dish. Transfer to a serving bowl or plate and serve warm or at room temperature.