Place a 10-inch nonstick pan with an ovenproof handle in the top third of the preheated oven. Allow the pan to heat for 3 to 5 minutes.
Brush 1 of each of the bread slices with a slight coating of olive oil.
Build the sandwiches by placing 2 slices of bread oil side down. Top each slice with half the cheese, spreading it evenly. Season with pepper and a few grains of salt. Top each sandwich with 1 of the remaining slices of bread, oil side up.
Place the sandwiches in the heated pan and cook for 5 minutes. Turn them over and cook for 5 minutes more; remove the pan from the oven. Open the sandwiches and fill with the onion, tomato and arugula.