Dry roast the green moong dal on low flame for a few minutes until aromatic. Switch off the stove and allow it to cool.
Wash the dal well and soak it in water for at least 1 hour.
Add the soaked dal to a pressure cooker along with chopped onion, tomato, green chillies, tamarind, turmeric powder, asafoetida, salt, and water.
Pressure cook for 3 to 4 whistles or until the dal is soft and well cooked.
Once pressure releases, gently mash the cooked dal to get a soft kadaiyal texture.
Heat gingelly oil in a tempering pan. Add sliced onion and saute until golden and lightly crispy.
Pour the onion tempering over the mashed dal and mix well.
Serve hot with rice and ghee.