01 - Melt butter in a large pot over medium heat. Add leeks, onion, and garlic. Cook gently for 6-8 minutes until softened and fragrant, avoiding browning.
02 - Add diced potatoes to the pot and stir for 2 minutes to lightly coat in butter and begin cooking process.
03 - Pour in vegetable broth and water. Bring to a boil, then reduce heat to low and simmer uncovered for 20-25 minutes until potatoes are completely tender when pierced with a fork.
04 - Remove from heat. Puree using an immersion blender until completely smooth and velvety. Alternatively, blend in batches using a standard blender and return to pot.
05 - Stir in heavy cream, salt, pepper, and nutmeg. Reheat gently over low heat until warmed through, being careful not to boil. Taste and adjust seasoning as needed.
06 - Ladle hot soup into serving bowls. Garnish with chopped fresh chives or parsley if desired. Serve immediately while hot.