Chop the pineapple into big cubes and grind 4 rings of the pineapple into a fine puree. Keep it aside.
Chop 3 rings as cubes for garnishing and the tomatoes into big pieces.
Chop the tomato.
In a kadai, add oil and splutter mustard seeds. Once they crack, add jeera, red chilli , curry leaves and saute until they turn golden brown.
Now add chopped tomatoes, add little quantity of water and cook the tomatoes.
At this point, you can add salt, turmeric powder, rasam powder, jeera powder, asafoetida, pepper powder and allow to boil. Once the tomatoes are cooked, add the ground pineapple, required quantity of water and allow it to cook in low to medium flame.
When the rasam starts forming foam, it is done.
Garnish it with chopped pineapple cubes and chopped coriander.
Serve it with hot rice and papad.