the very first step is to prepare toor dal. Take a pressure cooker, transfer toor dal into that, add water and pressure cook it for 3-4 whistles. keep aside.
meanwhile, finely chop your tomatoes.
take a pan and add tomatoes.
add fresh coriander and ginger.
also add chopped green chilies and mix.
add turmeric powder and water.
also, add salt according to your taste.
bring this mixture to boil for 7 mins or until the tomatoes are cooked well.
add black pepper powder according to your taste.
add boiled toor dal.
mash all the mixture a bit if you want.
boil everything for 5 to 8 mins.
add some lemon juice. keep aside.
for tadka:
heat ghee in pan.
add mustard seeds and cumin.
add curry leaves and dry red chili.
when they crackle and your tadka is ready.
pour this tadka on the rasam and mix well.
serve hot with rice.
Notes
you can use grated tomatoes or fresh tomato puree instead of chopped tomatoes if you don't like tomato chunks.
adjust the black pepper powder according to your taste.