Add the spinach to a big stock pot, and add 1/4 cup water. Cover and cook until spinach is wilted.
Meanwhile, dice and fry the onion.
Puree the spinach, onion, and milk in a blender. Add 2 cups water, the cube of vegetable boillion, pepper to taste, and heat.
Serve with bread. Soup is very thin, with a mild but pleasant flavour.