01 - Melt butter in a large pot over medium heat. Add leeks, onion, and garlic; cook, stirring occasionally, until softened and translucent without browning, about 8 minutes.
02 - Add diced potatoes, bay leaf, salt, and pepper to the pot, stirring to combine evenly.
03 - Pour in the stock and bring to a boil. Reduce heat to low, cover, and simmer until potatoes are tender, approximately 20 minutes.
04 - Remove the bay leaf. Using an immersion blender, puree the soup until smooth, or blend carefully in batches with a countertop blender.
05 - Return the soup to the pot if necessary. Stir in milk or cream and warm gently over low heat without allowing it to boil.
06 - Adjust seasoning as needed. Ladle into bowls and garnish with fresh chives or parsley and a drizzle of cream if desired.