01 - Heat a large pot over medium heat and add a little 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 and release the garlic aroma.
03 - Pour in the vegetable broth and bring to a rolling boil. Reduce heat to low and simmer gently for 10 minutes, or until the potato pieces are fork-tender.
04 - Add the fresh spinach to the pot and cook for 2 to 3 minutes, just until the leaves are fully wilted and have turned a vibrant bright green.
05 - Remove the pot from heat. Using an immersion blender, puree the soup directly in the pot until completely smooth and velvety. Alternatively, transfer in batches to a countertop blender and blend until silky.
06 - Stir in the milk and season with ground nutmeg, salt, and black pepper. Gently reheat over low heat if necessary, but avoid boiling once the milk has been added.
07 - Ladle the hot soup into bowls. Garnish with a swirl of fresh cream, a handful of croutons or toasted bread, and a sprinkle of fresh herbs if desired.