01 - Slice each chicken breast horizontally to create 4 thin cutlets. Pound each cutlet to an even 1/2 inch thickness. Season both sides with salt and black pepper.
02 - Arrange three shallow bowls: one with flour, one with beaten eggs, and one with panko breadcrumbs.
03 - Dredge each cutlet in flour, then dip in beaten egg, and coat completely with panko breadcrumbs.
04 - Heat 1/2 inch vegetable oil in a large skillet over medium-high heat. Fry cutlets in batches for 2 to 3 minutes per side, until golden and cooked through. Drain on paper towels.
05 - Lightly toast the brioche or milk bread buns.
06 - Spread 1 tablespoon of Japanese mayonnaise on the bottom half of each bun. Top with shredded cabbage, cucumber slices, and tomato slices.
07 - Place one hot chicken cutlet on top of the vegetables. Drizzle with 1 tablespoon of tonkatsu sauce.
08 - Cover with the top bun and serve immediately while hot.