To make the marinated onions: place the sliced onions in a small bowl. Toss with the olive oil, red wine vinegar, oregano, and salt until well coated. Marinate for at least 30 minutes.
Slice the loaf of ciabatta in half length-wise.
In a small bowl, mix the mayonnaise and chili paste together until smooth. Spread the mayonnaise on the bottom half of the ciabatta.
Layer on the provolone cheese, mortadella, capocollo, genoa salami, and soppressata. Top with the thinly sliced tomatoes and sprinkle with flaky sea salt and cracked pepper.
Add the torn pieces of mozzarella and the marinated onions and top with the shredded iceberg and arugula. Drizzle with the dressing from the marinated onions and top with the top slice of ciabatta.
Cut the sandwich into 6-8 pieces and enjoy.