1. Prepare the leeks: Slice the leeks in half lengthwise and rinse well under running water to remove any grit. Slice them into thin rounds. 2. Sauté the vegetables: Heat the butter (or olive oil) in a large pot over medium heat. Add the chopped onion and garlic, cooking until softened, about 3-4 minutes. 3. Add the leeks and potatoes: Stir in the sliced leeks and diced potatoes, seasoning with salt and black pepper. Cook for another 5 minutes until the leeks start to soften. 4. Pour in the stock: Add the vegetable or chicken stock, bringing the mixture to a gentle simmer. Cover and let cook for 15-20 minutes, until the potatoes are tender. 5. Blend the soup: Use an immersion blender to blend the soup until smooth, or transfer to a blender in batches. If you prefer a chunkier texture, blend only half of the soup and mix it back in. 6. Finish with cream (optional): Stir in the heavy cream and a pinch of nutmeg for extra richness. Adjust seasoning to taste. 7. Serve: Ladle into bowls, garnish with fresh chives or parsley, and serve with crusty bread or croutons.