How to Make It
In a bowl, combine the onion, jalapeño, bell pepper, cilantro, olive oil, lemon juice and lemon zest and season with salt and pepper. Spread 2 tablespoons of the fresh onion relish on each kaiser roll half.
Set a large skillet over moderate heat and coat with a thin layer of vegetable oil. Place 2 roll halves in the skillet, relish side up, and top each with 2 mozzarella slices, 4 pork slices and 2 more mozzarella slices. Reduce the heat to low and cover each sandwich with another roll half, relish side down. Weight down the sandwiches with a medium skillet and cook until the bottoms are toasted and the bottom layer of cheese has melted, about 4 minutes. Turn the sandwiches, add a little more oil to the skillet and brown the second sides, about 4 minutes. Transfer to plates and repeat with the remaining ingredients to make 2 more sandwiches.