Heat oil in a non-stick pan. Add onion and sauté till light brown.
Dice potato. Add spinach puree to the onion in the pan and mix well. Add salt, black pepper powder and ¾ cup water. Mix well and cook for 2-3 minutes or till it comes to a boil.
Pour spinach mixture into a blender jar, add potato and 5-6 walnuts. Blend well.Add ½ cup milk and blend till smooth. Transfer soup into a deep non-stick pan on heat. Add remaining milk and mix. Adjust seasoning, mix and cook for 2-3 minutes.
Chop remaining walnuts. Pour the soup into a serving bowl, garnish with chopped walnuts and serve piping hot.