Season your chosen beef or chicken cutlets generously with salt and pepper on both sides. Set up a dredging station with flour, beaten egg, and panko breadcrumbs in separate shallow dishes. Dip each cutlet first into flour, then coat it in beaten egg before rolling it in panko breadcrumbs until fully covered.
Heat about half an inch of vegetable oil in a skillet over medium heat until shimmering. Carefully place each breaded cutlet into the hot oil and fry for about 3–4 minutes on each side until golden-brown and crispy. Transfer to a wire rack or paper towel-lined plate to drain excess oil.
In a small saucepan over low heat, combine ketchup, Worcestershire sauce, soy sauce, mirin (or rice vinegar), brown sugar, and Dijon mustard if using. Stir continuously while simmering for about 2–3 minutes until slightly thickened; then remove from heat.
Slice your fried cutlets into strips for serving. Divide cooked rice among individual bowls as a base layer; arrange sliced katsu on top and drizzle with tonkatsu sauce.
Add shredded cabbage or lettuce atop each serving along with any desired toppings like green onions, sesame seeds, pickled ginger, or a fried egg. Enjoy hot!