To make the Tonkatsu sauce: In a small saucepan, stir together the ketchup, Worcestershire sauce, sugar, soy sauce, mirin, garlic, ginger and mustard. Heat over medium-high heat and stirring regularly, bring to a boil. Reduce heat to low and simmer until sauce is reduced by about 1/4 (7-8 minutes). Let the sauce cool and refrigerate, if making head or, keep warm if you will be serving immediately. (Sauce can be made ahead and refrigerated for up to 1 week).
To make the Tonkatsu:
Prepare 4 plates. On one plate, combine the soy sauce, garlic and half of the salt and pepper. On another plate, whisk together the flour and the remaining salt and pepper. On the third plate, whisk together the eggs with the water. On the last plate, add the panko crumbs.
Add one pork piece to the soy sauce plate and toss to coat. Next, dip it into the flour mixture to coat. Shake off excess. Dip into the egg mixture, letting excess drip off. Dip into panko crumbs, patting to coat evenly. Place on a wire cooling rack. Continue until all the pork pieces are breaded.
In a non-stick skillet just large enough to hold all your pork, add oil until it just covers the bottom of the skillet in a thin, even layer. Heat oil to just a bit more than medium (but not all the way to medium-high). When hot, add the breaded pork to the skillet and cook until the underside is golden, then flip and fry the other side until golden. Remove to a cutting board. Allow to a couple of minutes while your prepare your bowls.
Fill bowls with hot cooked rice. Add some cucumber and carrot slices to one side and some shredded cabbage on the other side. When pork has cooled slightly, cut cooked pork pieces crosswise into 3/4-inch wide strips. Place pork slices in the centre of the bowls. Drizzle with some warm Tonkatsu sauce and garnish with some sesame seeds and sliced green onion.