Pressure cook half cup toor dal with around 1 cup of water and some turmeric powder for around 6 whistles or till it is completely cooked. Mash the cooked toor dal well and keep aside.
In a pan, add 1 teaspoon oil and on a low flame dry roast all ingredients mentioned in the sambar masala, starting with channa dal, urad dal, fenugreek and peppercorn, chillies and finally coconut and curry leaves. The ingredients need not be roasted much and have to just slightly change colour.
Cool the roasted ingredients and grind to a fine paste along with some water.
Chop the veggies in large chunks. In a pot, heat 2 teaspoon oil and first add the yellow pumpkin and roast for a minute.
Next add the other veggies (ash gourd, brinjal and tomato) and sauté for 1 minute. Add some turmeric powder and salt and little water and cover and cook for around 7-8 minutes.
Once the vegetables are around 60-70% cooked, add the tamarind extract from the tamarind which was soaked earlier. Let it cook for 2-3 minutes.
Now add the ground masala, asafoetida and cook for further 2-3 minutes.
Finally add the cooked and mashed toor dal, jaggery and some water and cover and cook for 8-10 minutes. Keep stirring in between as the masala tends to settle down and might burn if left unattended. The sambar tends to get thick after cooling down, so adjust the consistency by adding more water according to your preference.
For the tempering, heat 2 tablespoons on coconut oil in a separate kadai, and add the mustard seeds and urad dal. Once the mustard seeds start sputtering add the curry leaves and turn off the stove.
Add this tadka or tempering to the sambar and mix well and serve hot.