Peel potatoes and cut into medium size peieces. Peel onion and roughly chop it. Combine these two ingredients in a heavy based pot and boil on medium heat until tender (about 25 minutes). You may also pressure cook it (with 2 whistles).
Strain potatoes and onions, throw away water and puree the potatoes and onion mixture in an electric blender. Then add tomato puree and blend more.
Separately in a frying pan, heat butter and add red chilli powder, black pepper powder and salt. Add the puree and lemon juice to this and bring it to a boil. Reduce heat and slowly add the cornflour (dissolved in water) and continuously stir.
When the soup becomes thick switch off the gas.
Garnish with fresh coriander/cilantro leaves before serving.