You could use ready-made bought teriyaki sauce for this, or make your own by mixing 3 Tbs light soy sauce, 3 Tbs mirin
or sweet sherry, 1 Tbs sesame oil, 2 Tbs brown sugar and a Tbs grated ginger. Bring this to a boil in a small saucepan
and simmer gently for a few minutes, until it thickens slightly. Slice fillet steak thinly across the grain (it's easier
to do this if you chill it for half an hour or so in the freezer) and fry quickly in very hot olive oil, until just cooked,
then scoop out and set aside. Fry sliced mushrooms until they lose their moisture and start to turn golden brown at the edges,
then scoop these out and put with the steak. Now add the teriyaki sauce to the pan, bring this to a simmer and return the steak
and mushrooms, tossing in the sauce to coat everything. Season with freshly ground black pepper, add spring onions cut diagonally
into chunks, bring everything to a simmer and serve immediately. Meanwhile, in another pan, fry chopped ginger and thinly sliced
garlic for a minute, then add baby spinach leaves, season with a splash of soy sauce and toss until it just starts to wilt,
then garnish with chopped red chilli. Serve the teriyaki beef with the spinach and accompanied with some rice.