Heat oil in a large saucepan over medium heat. Add onion, garlic, rosemary, pepper; sprinkle with the salt, reduce heat to medium-low and cook, partially covered for 5 minutes, stirring occasionally, until onion is soft.

Stir in potatoes and cook, stirring occasionally, for 3 minutes. Pour in broth or water and bring to a simmer over medium heat, cover and cook until the potatoes are soft, about 15 minutes.

Stir in spinach and simmer until the greens are wilted and tender, about 5 minutes or so.

Puree the soup with an immersion blender, or in a regular blender until it is your desired consistency.  I like it totally smooth and creamy but you can leave it a little chunky if you want.  Add salt to taste. 

Serve the soup garnished with a pinch of nutmeg and a small spritz of lemon juice for extra flavor if desired.  I like the acid with the flavors of the soup.  I really think it takes it up a notch.