Cook the katsu chicken in the oven or air fryer as per packet instructions. While the chicken is cooking, place the katsu sachets in a bowl of hot water until defrosted.
In a small bowl, combine the shredded cabbage, mayonnaise, lemon juice and sesame seeds. Season with salt & pepper.
Spread 4 slices of bread with the butter. Add a layer of slaw on top of two slices, then top with the katsu chicken fillets. Drizzle over the katsu sauce, then sandwich together and cut in half. Remove the crusts if that’s your preference!