Take a deep kadai and add 1 cup of water (Add water if needed).
Then add chopped tomatoes, slitted green chilli, rasam powder, and curry leaves.
When it starts to boil add diced pineapple pieces and bring them to boil.
When it starts to boiling add the mashed dal to this.
Do not allow this rasam to boil.
Then remove from heat and add lemon juice and salt.
Then season with mustard seeds and pour on top of rasam along with asafoetida.
Finally garnish with coriander leaves.
Serve hot with rice.