In a bowl, boil tomatoes with green chilli, salt to taste and water. Cover with a lid and cook on a medium heat for 10 minutes or till done / tomatoes are soft.
Add coconut paste and mix it well. Add 1/4 cup ( 50 ml ) water if required. Mix well and cook on a very low heat for 5 - 8 minutes ( till tomatoes are well mixed with coconut paste and gravy slightly thickens ). Color of the tomato curry will be bright yellow in color.
Heat oil in a pan. Add mustard seeds. When they begin to spluttering add cumin seeds, curry leaves and sliced onions. Fry on a low heat for 8 - 10 minutes or until fragrant and onions are golden brown in color.
Pour this seasoning over tomato curry and serve with rice or rotis.