Heat oil in a pan and fry the onions till they become soft.
Now add garlic and ginger and fry.
Roughly chop the tomatoes and add to the pan and fry for a minute.
Now chop the potatoes and add to the pan with salt, red chili powder and water.
Bring the soup to the boil, then cover and cook on low heat for 30 minutes.
Remove the lid and strain the soup through a fine sifter.
Return the pan to the stove and with a spoon clean off any scum that rises to the top.
Add the lemon juice and black pepper powder.
Heat for a few minutes and pour in a serving bowl.
Garnish with fresh coriander.
Serve hot.
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