Place the cashews in a blender with 1/2 cup water and blend into a very smooth paste. Unless you are doing this in a high powered blender, you might need to soak the cashews for 30 minutes first.
Heat the oil in a saucepan. Add the onions and a pinch of salt and pepper. Saute over medium-low heat, stirring frequently, until translucent.
Add the leeks and continue to cook for another couple of minutes. Add the potatoes and stir to mix.
Pour in the vegetable stock. You should have enough to cover the potatoes, so add more if needed. Bring the soup to a boil, cover, and cook until the potatoes are very tender.
Let the soup cool a bit, then transfer to a blender, in batches, to puree. Add more water if you need and don't overprocess-- let the blender run for just 5-6 seconds at a time. Potatoes can turn into a gummy mess if overprocessed, so make sure you have enough liquid in your blender.
Return the pureed soup back to the saucepan and bring to a boil. Add the cashew cream and add more salt and ground black pepper, if needed.
Serve hot, garnish with more cashew cream or some parsley if you wish.