In a pressure cooker, add tuvar dal along with a pinch of turmeric and salt. Cook for four to five whistles until the dal becomes soft. Once cooked, mash the dal thoroughly and set it aside to cool and absorb the flavours.
Coarsely grind cumin seeds, black pepper, and garlic together using a mortar and pestle or a grinder. This freshly ground spice mix will give the rasam its signature aroma and flavour. Set this aside for later use.
Take about 80 per cent of the chopped pineapples and blend them into a smooth puree using a blender. Make sure to leave some chunks of pineapple aside to add at a later stage, which will give the rasam extra texture and sweetness.
Coarsely grind tomatoes, ensuring they are not too smooth. This step helps in releasing the tangy juices of the tomatoes, which form the base of the rasam. Set the ground tomatoes aside, ready to be added to the pot.
Heat a tablespoon of oil in a pan and add mustard seeds. Let them splutter and pop, releasing their fragrance. Then add dried red chilli, fresh curry leaves, and a pinch of asafoetida (hing) for additional flavour. Stir in the ground jeera, black pepper, and garlic mixture, sautéing everything together for about a minute until aromatic.
Add the coarsely ground tomatoes to the pan and cook them until they soften and release their juices. Next, stir in the pineapple puree along with the mashed dal. Add about 1.5 cups of water, bring the rasam to a boil, and then drop in the reserved pineapple cubes. Let everything simmer for a few minutes. Garnish with freshly chopped coriander leaves and serve this hot, tangy, and flavourful pineapple rasam.