Step 1: Prepare the Chicken. If using chicken breasts, place each breast between two pieces of plastic wrap and pound them with a meat mallet or rolling pin until they are about 1/2-inch thick. This ensures even cooking. Pat the chicken dry with paper towels; this helps the breading adhere better.
Step 2: Season the Chicken. Lightly season both sides of the chicken cutlets with salt and black pepper.
Step 3: Set Up the Breading Station. Prepare three shallow dishes. In the first, add all-purpose flour. In the second, whisk the eggs until well combined. In the third, place the panko breadcrumbs.
Step 4: Bread the Chicken. Dredge each chicken cutlet first in the flour, shaking off any excess. Then dip it into the beaten egg, ensuring it's fully coated. Finally, press it firmly into the panko breadcrumbs, ensuring every surface is generously covered.
Step 5: Prepare the Tonkatsu Sauce. In a small bowl, whisk together the ketchup, Worcestershire sauce, soy sauce, sugar, and mirin (if using) until smooth. Taste and adjust seasonings as needed. Set aside.
Step 6: Fry the Katsu. Heat about 1 inch of vegetable oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat to 350-375°F (175-190°C). Carefully place 1-2 breaded chicken cutlets into the hot oil, ensuring not to overcrowd the pan. Fry for 3-4 minutes per side, or until deep golden brown and cooked through (internal temperature of 165°F / 74°C). Remove the katsu and place it on a wire rack set over a baking sheet to drain excess oil.
Step 7: Assemble the Bowls. Slice the crispy chicken katsu into 1/2-inch strips. Serve immediately over a bed of warm steamed rice. Drizzle generously with the homemade Tonkatsu sauce. Garnish with shredded cabbage, pickled ginger, and thinly sliced green onions.