rinse and then add 1/4 cup tuvar dal or pigeon pea lentils in a pressure cooker along with a pinch of turmeric powder.
add 3/4 to 1 cup water and pressure cook for 8 to 9 whistles or till the dal has softened and cooked really well.
when the pressure settles down on its own, check the dal and mash with a spoon or wired whisk. keep the dal aside.
in a small grinder or chutney grinder, take 1/3 cup chopped coriander, 2 tsp cumin seeds, 1/2 tsp black pepper, 1 or 2 green chilies (chopped), 1/2 inch ginger (roughly chopped) and 4 to 5 garlic cloves (roughly chopped).
without adding water grind to a coarse paste. keep aside.
in a pan or pot, heat 1 tbsp sesame oil or any oil and add 1/2 tsp mustard seeds. saute till the mustard seeds start crackling.
then add 12 to 15 curry leaves and 1 to 2 dry red chilies. saute till the red chilies change their color and darken a bit. don't burn them.
now add the ground coarse paste along with a pinch of asafoetida/hing and 1/4 tsp turmeric powder. stir very well.
add the cooked dal and add 1 to 1.5 cups water. stir well.
season with salt and again stir.
on a low to medium flame simmer the rasam for 5 to 6 minutes.
switch off the flame. wait for 1 minute and then add 2.5 to 3 tbsp lemon juice. you can add more or less as required and as per your taste. instead of lemon you can also add lime juice.