In a saucepot on medium heat, add the oil and wait for it to shimmer. Add the onion and garlic, and cook until the onion is soft, about 5 minutes. Add the spinach and stir every once in a while, until the spinach wilts. Keep adding more spinach, and keep stirring, until all of the spinach has shriveled.
Pour in the vegetable broth and bring the mixture to a boil.
Once the mixture has boiled, add everything into a blender and blend until smooth.
Return the spinach soup back into the pot and mix in the milk.
Serve in individual bowls and top each bowl with some croutons.
