1. Put potato and onion pieces in a pan and cook on medium heat till tender(25 minutes). You can also pressure cook it (2 whistles).

2. Puree the potato-onion mixture in an electric blender. Add tomato puree and blend once.

3. Heat butter in a pan. Add red chilli powder, black pepper powder, salt.

4. Add the puree and lemon juice and bring it to boil. Reduce the heat. Slowly add cornflour (dissolved in water) and continuously stir. When the soup becomes thick switch off the gas.

Serve hot. Garnish it with finely chopped coriander leaves.