01 - Heat a large pot over medium heat and add butter or olive oil. Sauté the finely chopped onion until soft and translucent, about 5 minutes.
02 - Add the minced garlic and diced potato to the pot, stirring continuously for 2 minutes to coat evenly.
03 - Pour in the vegetable broth and bring to a boil. Reduce heat to low and simmer for 10 minutes, or until the potato pieces are fork-tender.
04 - Add the fresh spinach and cook for 2 to 3 minutes until fully wilted and vibrant green in color.
05 - Remove the pot from heat. Using an immersion blender, puree the soup directly in the pot until smooth and velvety. Alternatively, blend in batches using a countertop blender.
06 - Stir in the milk and season with ground nutmeg, salt, and black pepper. Gently reheat if needed, but do not bring to a boil after the milk has been added.
07 - Ladle the hot soup into bowls. Garnish with a swirl of cream, croutons, and fresh herbs if desired. Serve immediately.