Lightly season both sides of the chicken cutlets with salt and pepper and sprinkle with flour. Dip the chicken into the lightly beaten egg and then into the panko, making sure that all sides are coated.
Heat up half an inch of oil in a large frying pan over medium high heat. Fry the chicken until golden brown and cooked through, flipping once, about 2 minutes per side. Drain on paper towels.
Spread Kewpie mayo on one slice of bread and Japanese mustard on the other. Add the chicken to one slice of bread and drizzle with tonkatsu sauce, top with the other slice of bread. Slice off the crusts, if desired and enjoy immediately.
