Wash tuvar dal in water 3-4 times.
Add 2 cups warm water and let the dal soak for 20-30 minutes.
Put the dal along with the water and turmeric powder in a pressure cooker and let it cook for 2-3 whistles. Alternately cook it in the Instant Pot at pressure cook mode for 12 minutes at normal low setting.
Once it's done, let the pressure from the cooker or IP reduce naturally. Open the cooker or Instant Pot.
Mash the dal lightly.
Add the chopped or pureed tomatoes, 3-4 cups of water and salt.
Mix and let the rasam simmer over medium to low heat till the tomato gets cooked and becomes soft.
Add rasam powder, jaggery and tamarind paste. Mix well. Simmer for another 10-15 minutes.