Place the chicken on a large cutting board, cover with plastic wrap, then pound with a meat tenderizer till the chicken is at approximately ½-inch thickness.
Marinade chicken in a large Ziploc bag with Worcestershire sauce, soy sauce, salt, and pepper for about 1 - 3 hours.
Place beaten eggs, flour, and panko breadcrumbs in three separate shallow dishes.
Heat up cooking oil in a frying pan over medium-high heat (about ¼ to ½ inch of oil; enough to shallow fry the chicken). Make sure that the oil is hot enough before frying the chicken.
Coat the chicken in a thin layer of flour (shake off the excess), dip into the egg, then coat in panko breadcrumbs.
Fry the chicken in small batches until golden brown (about 2 minutes on each side). Place the fried chicken on a cooling rack over a baking sheet, or on oil-absorbing paper.
In a small bowl, prepare the tonkatsu sauce by combining tomato ketchup, Worcestershire sauce, oyster sauce and ground mustard. Mix well.
Serve the chicken katsu with steamed rice and tonkatsu sauce.
Notes
Traditionally, Chicken Katsu is not marinated in Worcestershire sauce and soy sauce so you can omit this step if you wish. I just think it adds so much flavor to the chicken! Instead of chicken thighs, you can use chicken breast (cut into half horizontally) or chicken tenders.