Spread 1 Tbsp. pesto onto insides of both halves of each focaccia slice.
Build sandwich in following order: Bottom focaccia slice, 1½ oz. smoked mozzarella slice, 3 oz. chicken shreds, ¼ cup of arugula, 1 oz. pickled red onion, 1½ oz. of additional smoked mozzarella, top half of focaccia.
Generously grease griddle and toast sandwich on both sides until cheese is melted.