You can treat Pak Choi and Tatsoi as a Cut & Come Again crop.
Depending on when you planted it this autumn, it may be big enough to start harvesting now, and if so you can hope to get another crop from it again before Christmas if you simply cut the plant to about 3cm from the ground. It should re-sprout fairly quickly from the baby leaves in the centre, especially if the weather stays mild.
If it isn’t big enough to harvest yet, you can leave it to mature into a full head and harvest the entire thing in a few weeks time.