Heat olive oil on sauté mode of Instant Pot for about 5 minutes. Once the oil is glistening hot, add the onions and cook for about 10 minutes, stirring occasionally.
Add the potatoes, cashews, garlic, chicken stock, and nutritional yeast; stir to combine.
Cover and switch Instant Pot to soup/stew mode, making sure the vent is closed. Once the cycle is complete, either let the pressure release naturally or switch the vent open to release it quickly.
Using an immersion blender (or regular blender), carefully purée the hot soup until completely smooth. Be careful not to over-blend or soup will become gummy. Add the salt and freshly ground black pepper, to taste.