In a large cast iron pan over medium high heat, add olive oil. Cut the focaccia loaf in half lengthwise and place each side cut side down into the pan for 2-3 minutes to toast
Remove the loaf sides from the pan and spread pesto on the toasted insides of each half.
Layer the bottom half with all of the mortadella, then top with the burrata balls (I used 4 small ones). Split the burrata open and spread the gooey, milky insides over the entire sandwich
Top with a generous amount of arugula. Drizzle the top of the arugula with olive oil, and crack some black pepper over that.
Place the top of the loaf on the sandwich and press down. Cute into 4 squares/rectangles and serve immediately.
