(Optional) Toast the panko and the oil in a frying pan over medium heat, until the panko is golden brown (less than 5 minutes). Transfer the panko to a shallow dish and allow to cool. If you skip this step, the air frying / baking will not cause the panko to turn to a nice golden brown color. It will remain a light color.
Prepare a shallow dish with flour. Prepare another shallow dish with beaten egg. Line a baking sheet with foil. Place a rack (air fryer rack or baking rack) or top of the baking sheet.
Cut the pork tenderloin cross-wise into ½ inch medallions. Smash the medallions with the palm of your hands so that there is greater surface area. Sprinkle both sides of the medallions with salt and pepper.
Dredge the pork medallions in the flour and pat off any excess. Dip into the beaten egg and shake off any excess. Press the medallion firmly into the panko to coat. Flip and coat the other side.
Place the pork on the prepared rack and air fry for 15 minutes at 350F or bake in the oven at 400F for 20 minutes.
Cut each medallion into 1 inch strips and serve with ketchup or tonkatsu sauce!