Another look.

Ingredients
Small lime size tamarind (About 1/2 cup juice extracted)
Pinch of asafoetida
Salt
1 tbsp rasam powder (Adjust to taste)
1 tomato chopped
2 tbsp toor dal cooked (Thuvaram Paruppu)
Curry and coriander leaves for garnish
1/2 tsp oil
1/2 tsp mustard seeds
2 red chillies

Method
Add about 2 cups of water to the tamarind water and add asafoetida.

Add salt.

Add rasam powder.

Add tomatoes.

Bring to a boil.

Simmer for a couple of minutes.

Turn the stove off and garnish with curry and coriander leaves.

Do the tadka. Heat 1/2 tsp of oil and splutter mustard seeds. Add red chillies.

Add the tadka to the rasam.