Roughly chop pineapples into big chunks and keep aside. Finely chop tomatoes.
Rinse and pressure cook 1/4 cup of Tur dal or split pigeon peas with enough water (read notes below) until lentils are soft and mushy. Keep aside to cool.
Heat a heavy bottom pan, add the pineapple chunks, chopped tomatoes and water. Cook for 5 minutes.
Add rasam powder, turmeric and salt to taste. Mix well.
Cook until pineapples and tomatoes are slightly mushy.
Add cooked lentils and water to adjust consistency. Simmer rasam for 5 minutes.
Next add jaggery, mix. Taste check, adjust spices at this point. Garnish with finely chopped coriander and switch off the flame.
Heat a tempering pan, add ghee, mustard seeds and whole red chilies. Add hing or asafoetida once mustard crackles and switch off heat.
Add the tempering to the rasam, mix, cover and close the lid for 5 mins. Serve rasam piping hot.