Place the pork shoulder in between two sheets of plastic wrap. Using a rolling pin or meat mallet, pound the pork into 1/4 inch. Next, season with salt on both sides.
For the dredge, combine 1/4 cup all-purpose flour, 1/8 tsp salt, and 1/2 tsp onion powder in a shallow plate or bowl. Whisk together until well combined.
For the egg batter, crack the eggs into a shallow plate or bowl. Add 1/2 tsp salt, 1/3 cup all-purpose flour, and water. Mix until thoroughly combined.
For the breading, combine panko breadcrumbs and 1/8 tsp salt in a shallow plate or bowl. Mix together until combined.
Preheat the air fryer to 360F or 182C.
Coat the pork shoulder in the dredge, then transfer into the egg batter, and finally into the breading. Make sure the pork is coated on all sides at each stage.
Arrange the pork in the air fryer basket or tray in one layer, making sure there’s ample space in between. To make the katsu extra crispy, spray generously with cooking oil spray on both sides.
Air fry for 25 to 30 minutes or until the internal temperature reaches 145F or 63C. Be sure to flip halfway and spray both sides with cooking oil spray.
Remove from the air fryer and transfer to a cutting board. While the pork is resting and slightly cooling, prepare the tonkatsu sauce. In a small mixing bowl, mix together ketchup, Worcestershire sauce, oyster sauce, and sugar. Stir to combine well.
Slice the pork into 1-inch thick pieces or your desired thickness. Serve with a side of freshly cooked rice, shredded cabbage, and tonkatsu sauce. Enjoy!