Slice each chicken breast horizontally into two thinner pieces. Place the pieces between plastic wrap or parchment and gently pound them to an even thickness of about ½ inch (1 to 1¼ cm), trimming any uneven edges.

Season both sides of every chicken piece with the kosher salt and freshly ground black pepper.

Prepare three shallow dishes: place the flour in the first, beat the eggs with the water or milk in the second, and add the panko to the third.

Coat each chicken piece in flour and shake away the excess. Dip it into the egg mixture, then press it firmly into the panko so the crumbs adhere evenly.

Heat enough neutral oil to cover the skillet by about ½ inch over medium to medium-high heat. Bring it to approximately 340 to 350°F (170 to 175°C); a breadcrumb should sizzle immediately without the oil smoking.

Fry the breaded chicken in batches without crowding the pan for about 3 to 4 minutes per side. Turn once and continue until the crust is deeply golden and the thickest part reaches 165°F (74°C), adding another minute or two for thicker pieces if needed.

Transfer the cooked cutlets to a wire rack or paper towels and let them rest for 2 to 3 minutes. Slice into strips after resting so the crust stays crisp and the juices remain in the chicken.

Serve the chicken katsu with cooked white rice, finely shredded green cabbage, and tonkatsu sauce. Add Japanese mustard or a lemon wedge if desired.