How to Make It
On a work surface, lay out 4 slices of the mortadella. Spoon some robiola in the center of each one and top it with a basil leaf. Fold the mortadella over the cheese, fold in the sides to form a neat packet and secure it with toothpicks. Repeat with the remaining mortadella, robiola and basil.
Light a grill or preheat a grill pan. Grill the mortadella packets over high heat for about 1 minute per side, until they are lightly charred and the cheese has melted. Discard the toothpicks.
In a bowl, toss the arugula with the olive oil and vinegar and season with salt. Transfer to a platter. Arrange the packets around the arugula and serve right away.