Breaded pork + pickled onion + white sandwich bread = one delicious sando.
Place the onion in a heatproof container. In a 2-cup liquid measuring cup, stir the water, both vinegars, the sugar, and kosher salt until the sugar and salt dissolve. Pour over the onion. Let the onion stand at room temperature for 1 hour.
Meanwhile, add enough oil to a deep pot to measure 3 inches. Heat over medium-high until a deep-fry thermometer registers 350˚. Set a wire rack inside a rimmed baking sheet.
Season the chops with salt and pepper. In one shallow bowl, place the flour. In a second shallow bowl, beat the eggs. In a third shallow bowl, add the panko. Dredge one chop in the flour; shake off the excess. Coat in the eggs; let the excess drip into the bowl. Dredge in the panko, pressing to adhere. Shake off the excess panko. Repeat with the remaining chops.
Working with one chop at a time, fry until golden and an instant-read thermometer registers 145° when inserted horizontally into the meat, about 5 minutes. Place on the wire rack. Season with salt.
Drizzle 1 tbsp. katsu sauce over each slice of bread. Build the sandwiches with the bread, cabbage, pork, and drained pickled onion. Cut in half.