How to Make It
Preheat a panini press or a griddle. Brush one side of each bread slice with butter and arrange buttered side down on a work surface. Spread each bread slice with 1 tablespoon of the mustard. Layer the mortadella, Robiola, arugula and pickled onions on 4 of the bread slices and close the sandwiches.
Grill the panini over moderate heat until browned and crisp outside, about 3 minutes in a press or 3 minutes per side on a griddle. Cut the panini in half and serve.