Heat the olive oil in a large pot over medium heat.
Add the chopped onion and sauté for about 5 minutes until it becomes translucent.
Stir in the minced garlic and cook for an additional 1-2 minutes until fragrant.
Add the diced potato and vegetable broth to the pot. Bring it to a boil, then reduce the heat and let it simmer for about 10 minutes, or until the potatoes are tender.
Add the chopped spinach to the pot and cook for another 5 minutes, until wilted.
Remove the pot from heat and let it cool slightly.
Using a blender or immersion blender, puree the soup until smooth.
Return the soup to the pot and stir in the salt, black pepper, nutmeg, and heavy cream (if using). Heat gently without boiling.
Remove from heat and stir in the lemon juice before serving.