Heat oil in a deep non stick pan. Chop onions roughly and add along with garlic and saute, Roughly chop spinach.
Crush potatoes and add to the pan. Add 2 cups water and salt and cook.Grind spinach with ¼ cup milk to smooth puree.
Add vegetable stock cube to the pan and continue to cook till onions are soft. Strain and let the residue cool. Then add it to the spinach and grind till smooth.
Put the strained stock back in the pan. Add the spinach puree and mix well. Add pepper powder and remaining milk, mix well and let it come to a boil.
Pour into a soup bowl and sprinkle some pepper powder. Serve piping hot.