In a small bowl or glass take a pinch of tamarind (of the size of a lemon) and soak it in warm water. Keep it aside.
In another bowl or glass take 3 tablespoons of Sambar powder and mix it with water, using a spoon, to make a nice smooth paste. Keep it aside.
Wash and clean 3/4 cup toor dal and cook it with 3 cups of water and 1 tsp salt, in a saucepan covered or in a pressure cooker for 4 whistles. Once it's done you can choose to mash the dal with a hand mixie for a smooth and shiny consistency or leave it as it is. Keep this aside.
While the dal cooks, wash, peel and cut vegetables of your liking. I like to use drumsticks, small brinjal, bottle gourd, snake gourd, ivy gourd, South Indian cucumber, yam, potatoes, French beans, pumpkin, carrot and lady finger in my Sambar.
Cut and cook them together in a sequence in another saucepan with 2 cups of water. By sequence I mean, I first cut and add the hard vegetables like drumsticks, carrot, yam, potatoes, French beans into the water, then I cut and add all the gourds and finally I cut and add the lady finger too.
This way none of the vegetables are overcooked or slimy. Once done, keep the cooked vegetables aside.
Next in a heavy bottom vessel, pour 2 tablespoons of coconut oil. Once hot, add mustard seeds, some curry leaves and a dry red chilli (optional).
When it starts to splutter, add the chopped onions and sauté.
When the onions turn translucent, add the chopped tomatoes and sauté for a minute. Now add the sambar powder paste we had prepared earlier. Add to this the squeezed extract from the soaked tamarind. Pour about 1-2 cups water.
Cover the vessel and allow the mixture to boil for a few minutes. If it starts to stick at the bottom, add more water and mix again. This step is crucial for the masalas to cook well so give it at least 6-8 minutes.
Next, add the cooked toor dal and vegetables and mix this concoction. Add more water if required as per your preferred consistency. Check for salt.
Cook for another 5 minutes and turn the flame off. Add chopped coriander leaves for garnish.
Serve it with idly, rice, dosa.