01 -
If you're using mayonnaise, spread a thin layer on one side of each bread slice - this adds extra richness but is totally optional. Layer the mozzarella slices evenly between two pieces of bread to create 2 sandwiches. Make sure the cheese doesn't hang over the edges.
02 -
In a shallow bowl, whisk together the eggs and milk, then season with salt and pepper. In another shallow bowl, combine the flour (and panko if you're using it) with more salt and pepper. Line a small tray with parchment paper - this is where your coated sandwiches will rest.
03 -
Working with one sandwich at a time, dip it completely in the egg-milk mixture, making sure all sides are coated. Then transfer it to the flour mixture, pressing gently to coat all sides evenly. Place the coated sandwich on your parchment-lined tray and repeat with the second sandwich.
04 -
Refrigerate the coated sandwiches for 20 minutes - this helps the coating set and prevents it from falling off during frying. Meanwhile, heat your oil in a cast iron skillet or heavy pot until it's about 1 inch deep and reaches 350°F.
05 -
Carefully lower the chilled sandwiches into the hot oil using a spatula. Fry for about 2 minutes on one side until golden brown, then flip and fry for another 2 minutes. The coating should be crispy and the cheese should be melted and gooey inside.
06 -
Transfer the fried sandwiches to a cutting board and let them cool for just a minute. Use a sharp serrated knife to cut through carefully - cut halfway through from one side, then flip and cut from the other side, stopping before you cut through the melted cheese center. This keeps all that gorgeous cheese from oozing out. Serve immediately while hot!