Pour French onion soup into a medium sauce pan and place over medium high heat.

Add meatballs and simmer covered for 5 minutes.

While meatballs heat, arrange rolls on a baking sheet, cut sides facing up.

Remove meatballs from heat and place 4 meatballs on each roll half.

Using a slotted spoon, cover the meatballs with onions from soup and 1 piece of cheese on top of each.

Toast sandwiches under a broiler or in a toaster oven until cheese is melted, about 1 minute.

Remove sandwiches from broiler or toaster oven and top each sandwich with ½ cup of arugula and ½ cup of liquid from soup. Serve immediately.