To make the tonkatsu sauce: In a small saucepan, bring the ketchup, Worcestershire sauce, sugar, soy sauce, mirin, garlic, ginger and mustard to a boil. Reduce heat to low and simmer until reduced to about 3/4 cup, about 10 minutes. Let the sauce cool, but keep warm if you will be serving immediately. (Sauce can be made ahead and refrigerated for up to 1 week).
Meanwhile, prepare 4 shallow bowls or deep plates. In one bowl, combine the soy sauce, garlic and half of the salt and pepper.
In another bowl, whisk together the flour and remaining salt and pepper. In another bowl, whisk eggs with 2 Tbsp. water. In the last bowl, add the panko crumbs.
Pat pork dry, then pass through the soy sauce and garlic, allowing excess to drip off. Next, press pork into the flour mixture to coat. Shake off excess. Dip into egg mixture, letting excess drip off. Finally, press into panko crumbs, patting to coat evenly. Place on a cooling rack and continue until all the pork pieces are breaded.
In a deep skillet or Dutch oven, heat about 1 inch of oil over medium-high heat. Cook the pork, two or three pieces at a time (don't over-crowd), turning once, until deep golden on both sides.Remove to a cooling rack to drain. If making ahead, allow to cool slightly, then place on a plate, cover loosely with plastic wrap and refrigerate.
To serve: Butter bread slices lightly. Place on fried pork piece in the middle of the bread. Spoon some tonkatsu sauce over-top. Sprinkle with shredded cabbage. Place second piece of bread on top. Trim the bread of crusts, then cut the remaining sandwich in half and serve, cut-side up.