Marinate the onions by combining sliced red onion with olive oil, red wine vinegar, dried oregano, and salt. Let sit for 30 minutes.

Prepare the spicy mayo by blending mayonnaise with Calabrian chili paste until smooth.

Slice ciabatta loaf in half lengthwise.

Spread the spicy mayo on both cut sides of the ciabatta.

Layer provolone cheese, then mortadella, capocollo, Genoa salami, and soppressata on the bottom half.

Add sliced tomatoes, seasoning with salt and pepper, followed by mozzarella, marinated onions, lettuce, and arugula.

Drizzle remaining marinated onion dressing over the sandwich, and place the top half back on.

Cut the sandwich into 6-8 individual pieces and serve.