Dry roast it over medium-low heat (on a number scale of 4) for about 6 to 8 minutes or until the horse gram turns deep brown/red. As you can see in the step 1 picture, horse gram lentils come in different shades of brown. The light brown ones should turn golden brown, and the darker shade lentils will turn deep brown. Don't let the lentils turn black. As you roast, you will hear the popping sound; you can turn it off at that stage and always roast in medium-low or even low heat.
Transfer the roasted dal to a bowl and add water. Let it soak for about 15 mts.
Discard the water and add 2 cups of fresh water, and pressure cook the roasted horsegram for 4 to 5 whistles. Let the pressure subside naturally and set it aside.
Reserve the stock water.
Take two to three tbsps of the cooked dal and grind it coarsely like the one below.
Prepare the rasam
Add the chopped tomato, crushed garlic, tamarind paste, salt, rasam powder, curry leaves, and turmeric powder.
Add 2 cups of water and mix well. Ensure you mix the tamarind nicely and that there aren't any lumps.
Bring this mixture to a gentle simmer.
Add half of the chopped cilantro and mix.
Let the rasam simmer for two more minutes.
Now add the ground horsegram, stock water (the water we used for cooking the horsegram), and ½ cup of water. Mix well. Check for taste, and if needed, add salt. At this stage, you can add jaggery if using.
Reduce the heat to medium-low and simmer the rasam until it's frothy. Do not boil the rasam after adding the dal.
Once the rasam becomes frothy like below, turn off the heat. In a separate tempering pan, heat the ghee or oil, and when it's hot, add the mustard seeds, cumin seeds, and asafoetida. When the mustard seeds splutter, add them to the rasam. Add the remaining cilantro and cover the rasam. Wait for 5 minutes for the cilantro and tempering flavors to infuse. Then enjoy the rasam warm.
Notes
If you use tamarind flesh, you can use a small lime-sized tamarind. Soak it in 1 cup of water and extract the juice. Use this juice along with water.
I used only 2 tbsp of the cooked dal for this rasam. You can use the full measure of cooked dal, but it will yield a thick rasam. So I always end up using two to three tbsp of the dal. But if you use the entire dal measure, increase the salt and add ½ tsp of ground pepper.
Garlic is optional. Also, you can add a dash of ground pepper for the additional kick.
Adjust the salt and rasam powder measure to taste.