Heat the oven to 200C/180C fan/gas 6 and put the coriander seeds on a plate. Score the skin of the duck breasts as many times as you can with a small sharp knife, then season with salt and press the skin into the coriander seeds. Heat an ovenproof frying pan and add the breasts, skin-side down. Put a heavy pan on top to weigh them down, and cook for 7-8 mins to render the fat, occasionally draining off the excess.
Add the plums and pak choi to the pan, flip the breasts over and add half the stock. Transfer to the oven and cook for 4-5 mins. Remove the duck breasts from the pan and transfer to a plate to rest along with the pak choi.
Return the pan with the plums to the heat and add the honey, soy, vinegar and remaining stock. Bring to the boil and continue to cook until syrupy. Carve each duck breast into four chunks. Arrange two halves of pak choi over each plate, and nestle the chunks of duck breast and the plums among the greens. Drizzle with the sauce, then sprinkle over the chilli flakes.