Very thinly slivered green cabbage or crisp lettucesuch as iceberg or romaine hearts
In a small bowl mix together the ketchup, Worcestershire, soy sauce, mirin, sugar, and mustard. Set aside.
Place the pork or chicken cutlets, one at a time, into a heavy-duty zipper top bag and use a rolling pin or meat mallet to gently pound them into cutlets 1/4-inch thick. Season with salt and pepper.
Place the flour, eggs, and panko in three separate shallow bowls. Dredge each cutlet one at a time in the flour, the eggs, then the panko (if you use one hand to work the meat into the dry ingredients, and one for the eggs, you will have cleaner hands). Set on a wire rack as each is coated.
Heat 1/4-inch of oil in a large skillet over medium high heat. Place two or three cutlets into the pan, making sure there is space between each of them so they can brown well. Cook for 2 to 3 minutes on each side, until the outer coating is nicely browned and the meat is cooked through. Transfer to a paper towel-lined plate as they finish. Repeat with the remaining cutlets, adding more oil as needed and allowing it to become hot between batches. You can use 2 skillets if you want the cooking process to go more quickly.
Slice the katsu into thin strips and serve over hot rice with the cabbage or lettuce, lemon wedges and Tonkatsu sauce.