01 - Melt butter in a large pot over medium heat. Add sliced leeks and chopped onion, cook for 5–7 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, then pour in vegetable broth and water. Bring to a boil, reduce heat to low, and simmer covered for 20 minutes until potatoes are completely tender.
04 - Remove from heat. Using an immersion blender, purée until completely smooth and velvety. Alternatively, transfer to a countertop blender in batches, blending until smooth.
05 - Return puréed soup to low heat. Whisk in heavy cream, salt, pepper, and nutmeg if using. Heat gently for 2–3 minutes without boiling.
06 - Taste and adjust seasoning if needed. Ladle into warmed bowls, garnish with chopped herbs and a drizzle of cream. Serve immediately.