Season the pounded chicken breasts with salt and pepper.
Prepare a breading station with three shallow bowls: one with flour, one with beaten eggs, and one with panko breadcrumbs. Dredge the chicken in flour, dip into egg, then coat thoroughly in panko.
Heat about 1 inch of oil in a skillet to 350°F (175°C). Fry the chicken for 3–4 minutes per side until golden brown and cooked through. Transfer to a wire rack or paper towel to drain.
In a small bowl, whisk together all tonkatsu sauce ingredients until smooth. In another bowl, mix Japanese mayo, rice vinegar, sugar, and optional herbs.
Slice the crispy katsu into strips. Scoop rice into bowls, place sliced katsu on top, drizzle with tonkatsu sauce and mayo sauce, and add optional toppings like shredded cabbage, sesame seeds, or green onions.