Preheat a grill to medium heat. Smear both sides of all 4 pieces of bread with a thin layer of mayo.

Grill the bread on the first side for 3-5 minutes until grill marks form. Flip, and add a layer of american cheese, then tomato, salt and pepper, and mozzarella on the toasted sides of two of the pieces of bread.

Top the mozzarella with the other two pieces of bread. Flip the sandwiches one or two more times to melt the cheese, then let rest off the grill for a minute or two before serving.