Put the steaks into a large freezer bag. Add the soy and sliced garlic, seal the bag and gently massage the soy and garlic into the meat. Place in the fridge for a minimum of 2 hours or overnight.
After marinating the beef, remove it from the fridge. Remove the meat from the bag and brush off any garlic. Heat a griddle pan over a good heat until hot. Brush the steaks with sunflower oil and touch a corner of one steak into the pan and you should hear a nice sizzle noise. Add the steaks and sear for 20 seconds, lift and rotate the steaks 90˚. This will give a criss cross pattern. Alternatively sear for 40 seconds without rotating and turn over and repeat. If the steaks are thick, use tongs to hold the edges on the pan to colour them all over. In total this should take 4-5 minutes.
Warm the stock in a pan large enough to accommodate the steaks and bring to a simmer. Add the steaks to the stock and reduce the heat. The ideal temperature of the liquid is 80c to 84c. If you don’t have a thermometer, don’t panic. Look for steam rising from the liquid but with no sign of simmering and you should be bang on. Poach the steaks for about 8 minutes. If you have a thermometer, the steaks will be perfect when the core temperature is 46˚, this will take any guess work out of the dish.
Lift the cooked steaks from the pan onto a warm plate and leave to rest in a warm place. Pour the remaining liquid through a fine sieve into a measuring jug. Keep 500ml and pour the rest into a container and freeze for other sauces.
Add the vegetables and ginger to the remaining cooking stock and simmer for 3 minutes.
Heat a medium frying pan or wok until hot. Add the butter and swirl it around the pan and add the pak choi or spinach. Stir and add the water. Stir fry until the greens have wilted and all the water has evaporated. Remove from the heat and season with salt and pepper.
Taste the sauce, it shouldn’t need much, if any, salt and add the coriander leaves at the last moment.
To serve, place a mound of pak choi in the centre of four deep plates. Top with a steak and spoon over the liquid, arranging the vegetables around.