04Melt 1/2 stick of butter on griddle or in a separate skillet. Place roll halves, cut side down in butter and brown over medium-low heat until golden brown.
05To serve, spoon meat/onion mixture over bottom half of rolls. Spoon 1-2 tablespoons pan juice over the top. Lay two mozzarella slices on each sandwich, followed by two tomato slices. Top with a generous amount of arugula, then press the top half of the roll on top. Cut in half and serve immediately.