1. Preheat oven to 350°F (175°C).
2. Soak the sliced red onions in cold water for 10 minutes to mellow sharpness, then drain well.
3. Whisk together mayonnaise, red wine vinegar, oregano, crushed red pepper, minced garlic, salt, and pepper to form a dressing.
4. Toss shredded iceberg lettuce, soaked onions, and pepperoncini slices in the dressing; adjust seasoning as needed.
5. Slice the bread lengthwise and spread Calabrian chili paste evenly on both sides of the cut loaf.
6. Layer cheeses on one half of the bread; place the meats on the other half.
7. Bake the sandwich in the oven for about 5 minutes, or until the cheese melts and the meats warm through.
8. Remove from oven, top the meat side with the dressed salad mixture, then add tomato slices, fresh onion, and sprinkle with grated parmesan cheese.
9. Slice and serve immediately for best flavor and texture.