Puree the tomatoes ( or mash them) in bowl. Now add turmeric powder, salt, madras curry powder and cut hot green chili.
Cut the lemon and take the juices ( take the seeds and throw it away)
Crush the garlic in pestle.
Heat a pan add 1 cup of water, Tomato puree mixture, half of the garlic and cook until it almost boils.
Heat oil another pan ( or you can use the same pan by transferring the liquid ) add mustard seed until it splatter, now add the dry chili ( you can break into small pieces), curry leaves, coriander leaves and rest of the garlic and cook for 1 minute.
Now pour the boiled liquid into the pan and heat for another 1 min. Now add the lemon juice and cook for another minute. Stop cooking and Lemon rasam is ready.