finely chop the tomatoes,chilli, ginger, coriander leaves and keep aside.
take washed dal in a pressure cooker. add chopped tomatoes, green chilli, ginger, salt, turmeric and 3 cups of water, mix well and pressure cook on a high flame for 5-6 whistles.
when the pressure settles check if the dal is cooked well. whisk the dal so that the dal gets mashed well.
adjust the consistency by adding required water. check the salt and add more if required.
get to a boil and turn off the flame. make sure your rasam is pouring watery consistency.
prepare the tempering. heat oil in a small pan. add the mustard seeds, jeera, urad dal, hing. allow to splutter.
add some chopped coriander leaves, squeeze some lemon juice and pour the tempering over the rasam.
serve lemon rasam / kat saaru with hot steamed rice.