Place potatoes, onion, water and salt in a large saucepan. Bring to a boil, cover, and simmer until the potatoes are tender. Remove from the heat, add spinach and garlic cloves. Set aside.
Purée the veggies in their cooking water. I used an immersion blender. Stir in the milk. Adjust the salt, and add pepper to taste. Serve hot, topped with a sprinkle of nutmeg.