Soak the tamarind (about the size of a large marble or a small lemon) in 1 cup of water. It is best to make it into a ball and then soak in water, so that it is easily discarded once all the juice has been extracted.
Wash the dhal (pigeon pea lentils) and add the turmeric powder.Pressure cook till the dhal is soft, in 1 1/2 cups of water. Cook it for 2 whistles. Leave the cooker to release pressure on its own, as the dhal will continue to cook during that time.
Dice the tomato, mince curry leaves and strip the thyme leaves from their stems.
To make the rasam: Start once the cooker is ready to be opened.
Extract the juice of the tamarind by crushing it in the water in which it is soaked. Strain the tamarind extract into a bowl, checking that there are no pieces of tamarind pulp or seeds, and pour into a pan. Set on the stove and turn the stove on.
Add 1/2 cup of water to the tamarind pulp and squeeze out any remaining juice. Add to the pan on the stove. Discard the used tamarind pulp.
To the tamarind extract on the stove, add chopped tomatoes, rasam powder, cumin powder, pepper powder, asafoetida powder, a few curry leaves and salt. Curry leaves are very good for health but we generally discard them on the plate. I mince them so that they and their nutrients are consumed along with the rasam.
Add the thyme leaves. I have suggested 1 tablespoon for 3 cups of water, however you could first try with half a tablespoon and then add more depending on how strong you want the thyme flavours to be. If using dry thyme, then take just half of quantity suggested for fresh leaves.
Bring the thyme rasam to a boil and then reduce the stove to low flame. Let the rasam simmer for 5 minutes so that the raw taste of the chilli in the rasam powder disappears.
Take the cooked thowar dhal and mash it well in its own liquid.
Pour into the rasam with 1/4 glass of water. Bring to a boil, keeping it boiling for 2 minutes. Simmer on low flame for 3 minutes. Taste for correctness of salt.
Remove from the stove.
Keeping the stove on low flame, heat ghee in a small tempering pan, add mustard seeds and let them burst. Add cumin seeds and after 5 secs, add the chopped curry leaves and turn off the stove. I love the crackle and aroma of fresh curry leaves in hot ghee!
Pour the ghee tempering on the rasam and keep the pan covered.
Serve hot with rice and ghee, a dry curry and papad. Or serve hot as Soup
Add freshly squeezed lemon juice just before serving. Lemon does not take kindly to heating, so add the juice only to portion being served so that any left over rasam may be reheated and served with a dash of lemon juice later on.