Heat the oil in a pressure cooker (without lid) over medium heat. Add the onions and cook for quite a long time to caramelize. The exact timing will depend on how sweet your onions are. Mine weren't sweet at all so the process took about 40 minutes. The way to caramelize is to cook the onions slowly over medium heat - perhaps adding a bit of veggie broth if they start to stick to the pan. You're looking for a nice deep brown color both on the onions and on the pan- that's the sugar coming out of the onion.
After the onions are nice and brown and the bottom of the pan is looking very brown add the potatoes. Stir and cook for about 5 minutes - to give the potatoes a bit of that nice roasted flavor.
Cover with the veggie broth. Raise the heat to high and lock on the pressure cooker lid. Bring to high pressure, then immediately turn off the heat. Allow the pressure to reduce naturally. Remove lid.
Using an immersion blender, blend the soup thoroughly. Serve immediately - maybe with some nice crusty bread.