Preheat the oven to 450 degrees F.
In a heavy skillet, cook the onion and bell pepper in the oil over moderately low heat, stirring occasionally, until it is softened. While the onion and pepper are cooking, in a bowl stir together the salami, mozzarella, pepperoncini, oregano, and vinegar. Add the onion-pepper mixture.
Cut each piece of bread horizontally with a serrated knife without slicing all the way through, remove some of the soft inner white bread, and arrange the bread pieces cut sides up in a jelly-roll pan. Spread the bread with the olive paste. Spread the antipasto mixture on both sides of each piece of bread and bake the heros in preheated oven for 5 minutes. Season the heros with salt and pepper and re-form them, pressing the 2 sides together.
Recipe courtesy of Gourmet Magazine