Heat oil in a pan and add coconut. Roast it till it starts changing colour. Add sliced small onion (3), chilli and coriander powder, fenugreek seeds, asafoetida and chana dal. Keep roasting till the coconut becomes a rich dark brown colour. Make sure you keep stirring the coconut continuously, otherwise it may burn. Let it cool completely and then grind to a smooth paste.
Wash the toor dal and pressure cook it with 2.5 cups water for 4-5 whistles on high flame (refer notes). Keep it closed till pressure drops, around 15-20 mins. Open the pressure cooker and mash the dal nicely with a wooden spoon or a whisk.
Add vegetables, salt and 2 cups of water to the mashed dal and cook till the veggies are 3/4th done.
Soak the tamarind in 2-3 tbsp hot water and make a pulp. Add the tamarind pulp to the dal and veggies. Mix well.
Add 1/3 cup hot water to the ground coconut paste and mix well. Add this to the dal veggie mix. Stir well. Let it cook for 5-7 mins.
Do a taste test of sambar at this point and add more tamarind pulp, asafoetida or roasted fenugreek powder, if required. Also add more hot water, to adjust the consistency, if required.
Heat oil in a pan and splutter mustard seeds. Add sliced small onion(5), dry red chilli and curry leaves. When onion becomes golden brown, switch off the gas and add it to the sambar. Let it rest for 20-25 minutes, for ll the flavour to settle in.
Notes
Sometimes the dal needs to cook a bit longer. So adjust the cooking time accordingly. I prefer the dal to cook very well, so that it's easy to mash.You can use any veggies of your preference. Do a taste test of tamarind to see it's strength and adjust the qty accordingly. Also some tamarind tend to be extra salty, in that case be careful with the salt you add to the sambar.Keep in mind that sambar tends to thicken a lot, once it cools down. So adjust all the flavours and water qty accordingly.Sambar keeps well for 3-4 days in refrigerator. You may need to add extra water while reheating to adjust the consistency.