Melt butter in a large saucepan.

Add leeks, onions, and potatoes to the saucepan.

Stir the vegetables to coat them in butter.

Cover the saucepan and sweat the vegetables for 10-15 minutes over medium-low heat, stirring occasionally.

Add chicken stock and milk to the saucepan.

Bring the mixture to a boil.

Reduce heat, replace the lid, and simmer for 20 minutes.

Remove the saucepan from heat and carefully blend the soup until smooth using an immersion blender or a regular blender.

Return the blended soup to the saucepan.

Season with salt and pepper to taste.

Add heavy cream to taste for extra richness and creaminess.

Serve hot.