Pull the fat off the sirloin, finely slice the fat and place it in a cold non-stick frying pan. Put on a medium-high heat to crisp up while you peel and matchstick the ginger, then add that to crisp up, too. Cut off the sinew, then dice the steak into 3cm chunks and toss with the miso until well coated. Scoop the crispy fat and ginger out and put aside, then add the steak chunks to the pan. Cook for 4 minutes, tossing regularly, then drizzle in the honey and 1 tablespoon of red wine vinegar. Toss for 1 more minute until shiny and sticky.
Meanwhile, halve the pak choi, cook in a pan of boiling water for just 1 minute so they retain a bit of crunch, then drain well and plate up. Spoon over the steak and sticky juices from the pan, and finish with the reserved crispy bits.