Sauté the onion in the oil until it becomes translucent, then add the garlic, and continue to cook for 3-4 minutes, until the raw garlic smell has gone.
Add the leeks and potatoes, and give everything a good stir. Cover the pan, and allow everything to sweat for 10 minutes.
Add the stock, stir again to make sure everything gets mixed together, replace the lid, and raise the heat under the pan to bring the soup to a boil.
As soon as it starts boiling, turn the heat down, and allow the soup to simmer for around 30 minutes, or until all the vegetables are soft.
When it's ready, remove the pan from the heat, and blend everything together, until smooth.
Ladle into bowls, and finish with a grind of black pepper and a few snipped chives.
This potato and leek soup will keep for up to three days in an airtight container in the fridge, or three months in the freezer.