Heat the oil in the bottom of the pressure cooker over medium high heat. If your cooker has an insert, remove it and cook directly in the bottom of the pot. Add the onion and beef, and cook until browned on the outside.
Stir in the stock, water, carrots and salt, close the lid and secure the pressure regulator. Heat until you start to hear sizzling, then reduce the heat to medium, and set your timer for 20 minutes.
Meanwhile, place the potatoes in a saucepan with enough water to cover. Bring to the boil, and cook until tender, about 10 minutes. Drain. This whole process should take about the same amount of time as the rest of the stew.
When the 20 minutes are up, release the pressure from the cooker according to the manufacturer's instructions.
Remove the lid, and place the pot over medium heat. Bring to the boil. Stir the cornflour into a small amount of cold water until dissolved. Stir this into the stew and cook for a few minutes. Add the potatoes to the stew, or place them in serving dishes and ladle the stew over them.