Boil the potatoes till they're cooked and very soft. Peel and break them up into smaller pieces (shape or size doesn't really matter).
Heat the butter or oil in a soup pot, and add the garlic and the onions. Cook till the onions are translucent but do not brown them. Add the potatoes and cook for about 5 minutes (again, do not brown) and then add the milk and the stock (or water and stock cubes).
Stir and let it come to a boil. Then let it cool a bit.
In the meanwhile, remove the thick stems of the spinach leaves and place them one on top of another. Roll them up tightly into a tube, and holding this down, cut into thin slices. When they open up you will lovely thin ribbons of spinach.
Now blend the warm soup well until smooth. Return this to the pot and add the spinach ribbons as well to the pot. Cook for a couple of minutes and season with nutmeg, salt and pepper.
Divide among 4 bowls and serve hot with Broa de Milho as is done in Portugal, with Poi or Pao as in Goa. Actually any thick and crusty bread will do. If by some chance, you have leftover soup, refrigerate it and warm it up the next day and serve. A lot of people swear that Caldo Verde actually tastes better the next day!