In a medium-sized saucepan, melt the butter over medium heat and soften the onions until they soften. Next, add the leeks and cubed potatoes. Please do not allow any of the vegetables to brown, or they'll affect the colour of the soup. Season with salt.
Cover with the milk and vegetable stock and gently bring to a light boil, then reduce to a simmer. Cook for 10 minutes until the vegetables are tender.
Using a hand blender or a standing blender, blitz the soup until smooth. Wipe out the pan and return the soup to it. Stir in the cream and season to taste.
Serve with a lot of bread.