In a wide pan add a tsp of oil along with cumin seeds, mustard seeds, urad dal, chana dal, coriander seeds, red chillis and few curry leaves. Saute for a minute on medium flame till it turns fragrant. Do not burn the spices.
Now add some onions and garlic paste. Continue to saute till the raw smell disappears. Switch off the flame and add coconut. Saute just to make coconut warm and also to cool the mixture.
Transfer all the roasted spices into a small mixer jar. Add a piece of tamarind and garam masala powder. Add water as required and blend to smooth paste. keep aside.
In a large kadai add a tsp of oil and saute garlic paste, green chilli paste and onions for 2 minutes. Now add potato and lauki and fry for a minute. Add 1 cup of water.
Along with water also add turmeric and salt to taste. Cover and cook till potatoes and lauki are ¾th done.
Add prepared masala paste and give a good stir.
Once the masala comes to a boil add a cup of water and boil for 5-8 minutes.
Add cooked dal. Make sure your dal is cooked well. Also add 2-3 cups of water to get desired consistency.
Note:- you can more dals if you want to.
Add a small piece of jaggery. You can alternatively use sugar.
Add some salt if required. Give a good mix and boil for another 5 minutes. Switch off the stove.
Prepare the tempering. Add a tsp of oil along with mustard seeds, urad dal and hing.
Once they splutter, add few curry leaves.
pour the tempering over prepared sambar.
Add few chopped coriander leaves and give a good mix.
Serve hot with idli, dosa, medu vada or rice.