Place a small non-stick frying pan over medium heat; add olive oil, onion, garlic and mushrooms. Cook for 5 minutes or until softened and caramelised. Add miso, soy and kecap manis and pan fry until the sauce is absorbed into the mushrooms (approx. 5 mins). Set aside to cool.
In a small bowl mix together the kewpie and teriyaki until combined.
To serve, spread one slice of bread with mayo, add the mushrooms and then top with slaw. Add the ginger followed by the other slice of bread. Serve either fresh or toasted.