Pour two cups of water in a microwave-safe bowl. Set the cooking time to 5 minutes (the tomato skins should peel out).
Drain the tomato pieces and peel the skin. Don't throw the water away.
Cook the onion pieces and corn together in the same water. Drain the corn and onion in a vessel.
When the items have cooled, grind them in a mixie or liquidiser. Add chilli and tamarind paste while grinding.
Heat a kadhai and pour 1 tbsp of oil. Season with mustard, hing and turmeric powder. Add the ground paste and salt.
Boil on high flame for 5 minutes, stirring continuously. Continue to cook on sim for 10 minutes, stirring occasionally. The chutney should be semi-solid. When the chutney cools a bit, add lemon juice (optional).