01 - Melt unsalted butter in a large pot over medium heat. Add leeks and onion, stirring frequently, and sauté until soft but not browned, approximately 6 to 8 minutes.
02 - Add minced garlic to the vegetables and cook for 1 minute until fragrant.
03 - Stir in diced potatoes, salt, black pepper, and ground nutmeg if using. Pour in vegetable broth and increase heat to bring the mixture to a boil.
04 - Lower heat to a gentle simmer, cover the pot, and cook for 20 minutes or until potatoes are completely tender.
05 - Remove the pot from heat. Using an immersion blender, blend the soup until silky smooth, or transfer in batches to a standard blender for puréeing.
06 - Return the puréed soup to the pot if necessary. Stir in whole milk or heavy cream and gently reheat over low heat, taking care not to boil.
07 - Taste the soup and adjust salt and pepper as needed.
08 - Ladle hot soup into bowls and garnish with chopped fresh chives or parsley.