To make the marinade add the soy sauce, cooking sake and mirin to a small bowl and stir. Transfer the marinade mixture to a pan and bring to a gentle simmer for a few minutes before adding the chopped spring onion and garlic, set aside to cool (your spring onion and garlic are to remain quite raw).
Season your fillet beef with salt and pepper and leave it to rest for about 10 minutes at room temperature. Grab a medium-sized frying pan, add a splash of sesame oil and on medium heat evenly sear your beef for one minute on each side, it should be rare on the inside.
Let it rest on a chopping board for 5 minutes then slice your fillet very finely and lay on a long plate. Pour the marinade sauce over the beef and serve with a small dipping bowl of ponzu soy sauce. You can also cover and leave it to chill in the fridge if you want to serve it cold with a higher marinade infusion.