01 - Melt butter in a large pot over medium heat. Add sliced leeks and chopped onion; sauté for 5 minutes until softened and translucent, avoiding browning.
02 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to burn.
03 - Add diced potatoes to the pot along with salt, pepper, and nutmeg. Cook for 2 minutes, stirring constantly to coat vegetables evenly with butter and seasonings.
04 - Pour in vegetable broth and bring to a boil. Reduce heat to low, cover, and simmer for 20 minutes or until potatoes are completely tender when pierced with a fork.
05 - Remove from heat. Using an immersion blender, purée the soup directly in the pot until completely smooth and velvety. Alternatively, blend in batches using a countertop blender and return to pot.
06 - Stir in milk or heavy cream. Heat gently over low heat until warmed through, about 2-3 minutes. Do not boil. Taste and adjust seasoning with additional salt and pepper if needed.
07 - Ladle hot soup into bowls and garnish with chopped fresh chives or parsley. Serve immediately while hot.