How to Make It
In a small bowl, combine the yogurt with the scallion, horseradish and mustard. Season with salt.
Heat a panini press or griddle. Brush both sides of each bread slice with oil. Arrange 4 bread slices on a work surface and spread with the horseradish yogurt. Layer the mortadella and Robiola on top and close the sandwiches with the remaining 4 bread slices. Set them on the preheated press or griddle. If using a griddle, top the panini with a large baking sheet and weigh down with heavy cans. Cook the panini over moderate heat until browned and crisp outside: 3 minutes in a press, 3 minutes per side on a griddle. Cut the panini in half and serve hot.