Preheat the oven to 200C Fan.
For the chicken katsu: butterfly the chicken breast length ways to create two thin pieces.
Place the breadcrumbs, flour and egg into 3 separate bowls. Mix the sesame seeds through the breadcrumbs until well combined.
Place the chicken in the flour, turning to coat completely. Next, pop into the bowl with the egg, once again, turning to completely coat. Finally, pop into the bowl with the sesame-breadcrumbs, making sure it is well coated on all sides.
Place on a baking sheet, then brush each side with a little sunflower oil.
Bake in the oven for about 15-20 minutes, turning halfway through, or until golden and crispy on the outside and cooked all the way through on the inside (there should be no pink remaining) - the exact length of time will depend on the size, shape and thickness of your chicken katsu.
Meanwhile, make the tonkatsu sauce: in a small bowl, mix together the ketchup, runny honey, soy sauce, grated garlic and ginger. Taste and adjust the flavours, as necessary.
Next, make the mayonnaise: mix together the mayonnaise, rice vinegar and runny honey in a small bowl. Taste and adjust the flavours, as necessary.
To assemble your Chicken Katsu Sandwiches: spread a generous dollop of the mayonnaise on each slice of bread. Top with a spoonful of tonkatsu sauce and spread in a smooth and even layer. Top with a piece of crispy chicken and a large handful of shredded cabbage. Place the other slice of bread on top, slice in half and devour!