Combine all the ingredients in a saucepan over medium heat and bring to the boil, stirring occasionally.
Reduce the heat to low and simmer for 10–12 minutes, skimming off any foam that rises to the top.
Remove from the heat and keep warm.
Chicken Katsu
Cut the chicken thighs in half and place between two pieces of plastic wrap. Use a rolling pin or the side of a pan to pound them to half their original thickness.
Place the flour, eggs and panko crumbs in separate bowls.
Season the chicken with salt and pepper and dip in the flour to coat. Dip into the egg, then the crumbs.
Two-thirds fill a deep-fryer or large heavy-based saucepan with vegetable oil. Heat to 180ºC (350ºF), or until a cube of bread dropped into the oil turns golden brown in 15 seconds.
Add the chicken and cook for 5–6 minutes, or until golden brown and cooked all the way through. Drain on paper towel.
Cabbage Salad
Combine all the ingredients in a bowl and set aside.
To Serve
Spread the bread slices with the butter and katsu sauce.
Top half the slices with the chicken pieces and a little cabbage salad, then sandwich the other bread slices on top.
Cut the crusts off and cut the sandwiches in half. Serve immediately.