Step 1

Mix ketchup and Worcestershire sauce in a small bowl.

Step 2

Do Ahead: Sauce can be made 1 week ahead. Cover and chill.

Step 3

Freeze pork loin until firm around edges, about 1 hour. Slice pork ⅛" thick (or as thinly as you can). Place slices between 2 pieces of plastic wrap and gently pound until as thin as possible without tearing meat.

Step 4

Place eggs, panko, and flour in separate shallow bowls. Divide pork into 4 portions (you should have at least 4 slices per portion but may have more depending on how thinly you slice the meat). Working with 1 portion, season pork slices lightly on both sides with salt and stack, placing 2 shiso leaves, if desired, between any 2 slices. Keeping stack intact, dredge in flour, shaking off excess. Dip in eggs, letting excess drip off, then coat in panko, shaking off excess. Set on a wire rack set inside a rimmed baking sheet and repeat with remaining shiso leaves and stacks of pork.

Step 5

Heat 2 Tbsp. oil in a large skillet over medium-high. Cook 2 stacks of pork until golden brown and cooked through, about 3 minutes per side. Transfer back to wire rack. Wipe out skillet and repeat with remaining 2 Tbsp. oil and 2 stacks of pork. Serve pork with tonkatsu sauce.