Shouldn't need a pressure cooker for soup.
Assuming I have stock (cubes, stock pot/flavour pot things or the genuine home-made article) I would fry the onions as you suggest, add the other veg and make sure they've sweated a bit as well, the add stock. If the veg are small enough, preferably grated, should only take about 15-20 minutes to be done. Then blend. I'd probably add the coconut milk at the end and just warm through.
Using the pressure cooker probably does things too fast with no time for the flavours to develop.