Make the sauce. Mix together the mustard, ketchup, worcestershire sauce and soy sauce. Put aside.
Slice each chicken breast in half horizontally with a really sharp knife, then cut each in half width-ways. Season the chicken with salt and pepper on both sides.
Spread the panko breadcrumbs out on a plate and whisk the egg in a dish.
Dip each piece of chicken in the egg and then in the breadcrumbs until it is totally covered in the crumbs. Shake then dip in the egg and crumbs a second time. Place the crumbed chicken pieces on a large plate.
At this point, put your rice in to boil or steam while the chicken is cooking.
Pour enough oil into a frying pan (skillet) to barely cover the bottom and heat it up. When it is hot enough (test by dropping a few breadcrumbs into it and If it sizzles energetically, it's ready!), place a few pieces of the crumbed chicken at a time into the pan. Fry for about 3 minutes on each side or until golden brown.
Drain any excess oil from the chicken on paper towels, then slice each piece of chicken width-ways into strips about 1 inch thick.
Serve immediately on top of your cooked rice. Drizzle a little of the tonkatsu sauce over the top of the chicken and around the rice. Serve extra on the side for dipping!
