Ingredients:

Tomatoes                      : 3 medium size
Tur dal                          : 2 tbsp
Tamarind                      : small lemon size
Chilli pdr                      : 1tsp
Coriander pdr               : 1tsp                 
Turmeric pdr                : a pinch
Jaggery                        : a small piece
Ghee                            : 1tsp
Mustard seeds              : 1tsp
Cumin seeds                : a pinch
Asafoetida powder       : a pinch
Red chillies                  : 2 split into 2
Coriander leaves          : for garnishing

Method:

Pressure cook tur dal ( When you cook tur dal for sambhar, just take 2 tbsps of cooked dal along with  water). Mash 1 tomato and and mix it with the dal and keep aside.

Soak tamarind  in 3 cups of water for 30 mins and take the extract. Now add  chopped tomatoes(2),salt, turmeric pdr, chilli pdr, coriander pdr,  asafoetida and jaggery  to this and let it boil for 5 mins.   Finally add the dal and tomato mix to this and boil for 1 more min and remove from fire.  Heat ghee in a kadai and mustard seeds, red chillies and cumin seeds. when it splutters add this to the rasam and garnish with coriander leaves.