Heat oil in a medium skillet over medium heat, and sauté the onions until soft and begin to brown, about 8 to 10 minutes.
Remove the onions from the pan and fry the mortadella 30 seconds to 1 minute per side or until lightly browned.
Assemble the sandwich by adding the cheese followed by the onions and topped with the mortadella. Spread dressing on top half of the roll or sliced bread.
