Soak the onion slices in a large bowl of cold water, 15 minutes.
Meanwhile, split the bread lengthwise, then pull out some of the bread from the inside. Drizzle 2 tablespoons each vinegar and olive oil on the bottom half. Season with salt and pepper.
Layer the cheese and meat on the bottom half of the bread. Drain the onion and pat dry. Top the meat with the onion, lettuce, pepperoncini (if using) and tomatoes. Drizzle with 2 tablespoons each vinegar and olive oil and sprinkle with the oregano. Season generously with salt and pepper.
Drizzle the cut side of the bread top with the remaining 1 tablespoon each vinegar and olive oil, then place on top of the sandwich. Cut into 4 pieces.