Melt the butter in a large lidded saucepan, add the onion and garlic and fry gently for 5-6 minutes until softening. Stir in the potato and continue to cook gently for 1 minute.
Pour in the stock and simmer for 8-10 minutes until the potato starts to cook.
Pour in the milk and bring up to a simmer, then add the lemon zest and stir in half the spinach.
Cover and simmer for 15 minutes until the spinach has completely wilted down, then allow to cool for about 5 minutes.
Pour the soup into a blender or food processor, add the remaining spinach and process until silky smooth (you may need to do this in batches depending on the size of your machine).
Return to the pan and reheat. Taste and season with the grated Parmesan, salt, pepper and nutmeg. You may like to dilute the soup with a little extra stock if too thick.
Ladle the soup into bowls and cover it with Parmigiano Reggiano shavings.
*All information included in this recipe has been provided by Italia Formaggi.