Melt the butter in a large saucepan over medium heat. Add the sliced leeks and diced onion. Cook gently for 8-10 minutes until soft and sweet - do not let them brown.
Add the diced potatoes and stir to coat in the butter.
Pour in the stock, bring to the boil, then reduce the heat and simmer for 15-20 minutes until the potatoes are tender.
Remove from the heat and blend with a hand blender until smooth. You can leave it slightly chunky if you prefer.
Stir in the milk and season with salt and white pepper. Heat through gently without boiling.
Serve in warm bowls with a sprinkle of chives and some crusty bread.