Add the tamarind, ginger, sugar, garlic and chopped chilli to a small pan and heat over a medium heat until the sugar has just dissolved. Remove from the heat and stir in the coriander stalks. Put the steaks in a small bowl, add the marinade and mix together until the steaks are well coated. Leave to marinate for 2 hours.
Bring a medium pan of salted water to the boil and cook the rice according to the instructions on the packet. Heat a heavy-bottomed frying pan over a high heat until smoking, then cook for 2½ minutes on each side for medium rare or 3 minutes for medium. Remove from the heat, place on a board and loosely cover with foil. Leave to rest.
Put the oyster sauce, sesame oil, sesame seeds and miso paste into a small pan with a splash of water over a medium heat. Gently simmer for 1 minute, stirring until well combined. Bring a medium pan of water to the boil and cook the pak choi and spinach for 2-3 minutes until just tender, then drain, quarter lengthways and toss with the sauce. Slice the steak, then scatter over the sliced chilli, coriander leaves and spring onions and serve with the greens and rice.