Pressure cook the toor dal with turmeric powder. Mash it well and set it aside. Add the rasam powder, asafoetida, salt and tamarind to about 2 cups of water. Allow to boil until smell of rasam powder disappears. Cut pineapple and tomato to very fine pieces. Add this to the boiling mix. Allow to boil for another 3 minutes.
Mash the dhal well and add this to the above mix. Dilute the rasam to the desired quantity with slightly warm water. Allow the rasam to boil until a layer of froth form on top. Remove from fire and set aside. Add oil to a frying pan. Add mustard, toor dal and curry leaves to it. splutter. After spluttering. Pour rasam and mix.