finely chop tomatoes, onion, green chilli, garlic, coriander leaves and keep them aside.
in a pan add oil. add mustard seeds, curry leaves, red chillies, urad dal and hing.
once mustard seeds splutters, add garlic. fry them for a minute or two till the raw smell of garlic goes away.
now add chopped onion and fry till they turn golden brown.
now add tomatoes, chilli and fry on medium flame.
add sugar and cook tomatoes on a medium flame.
now add salt to taste.
cook tomatoes till they turn mushy. now add water till you make thin waatery or rasam consistence.
now add 2 tbsp of rasam powder
and mix well.
add chopped coriander leaves and turn off the flame.
your tomato onion rasam is ready. serve hot with steamed rice along with papad.