Slab Muffuletta

Ingredients

• 1 sheet pan focaccia (see note)
• 6 tablespoons finely chopped castelvetrano olives (or other green olives)
• 6 tablespoons finely chopped oil-cured black olives (or other black olives)
• 6 tablespoons finely chopped roasted bell peppers
• 3 stalks celery, roughly chopped, plus any leaves that might be attached
• 3 tablespoons drained capers
• 3/4 cup flat-leaf parsley, roughly chopped
• 5 tablespoons extra-virgin olive oil
• 3 tablespoons red wine vinegar
• 1 1/2 teaspoons dried oregano
• Kosher salt, to taste
• 1 pound sliced capicola
• 1 pound sliced soppressata
• 1 pound sliced mortadella
• 1 pound sliced provolone