Not had much luck in the spring as we always seem to get an early hot spell and they bolt, but I am right down south, so it might well be better elsewhere.
I normally grow oriental veg as a greenhouse crop to follow on after the summer stuff. The seeds get started off end August/beginning September and then as everything clears out, I pot them up into largish pots, usually with some of the compost I have used over summer with just some fresh stuff and some fertiliser added. I usually grow looser leafed stuff than pak choi, that I can take as a cut and come again crop over the autumn and winter. I’m sure it would work in greenhouse border soil and as a polytunnel crop, but you do have to be on guard against slugs and snails.
Seems early or late sowing are the answers.