Begin by peeling and dicing the potatoes.

Cut the leeks and rinse them thoroughly to remove any dirt.

In a large pot, melt the butter over medium heat.

Add the chopped onions and leeks to the pot and sauté until softened, about 5 minutes.

Stir in the diced potatoes and mix well.

Add the broth to the pot and bring it to a boil.

Reduce the heat and let the soup simmer for about 20 minutes, or until the potatoes are tender.

Use an immersion blender to purée the soup until smooth.