Remove from heat,stir in coriander powder,allow to cool and mix in turmeric powder.
Grind to a fine paste with 1 1/2 cups water.
Wash bittergourds,remove seeds,cut into 3-cm pieces and set aside.
Heat 1/2 tbsp oil in a pan,add bittergourds,shallots and green chillies, and saute till bittergourds turn brown.
Extract tamarind pulp and strain into pan.
Stir in coconut -spice paste,salt and 1 1/4 cups water.Raise heat,boil for 10 minutes and remove from heat.
Heat oil for tempering in a small pan and sprinkle in mustard seeds.When they start sputtering,toss in red chillies and fry for few moments till fragrant,shaking the pan occasionally.Add remaining ingredients and fry till shallots turn brown.
Pour contents of pan into curry,mix well and serve.
Variation:Theeyal can also be made with shallots,okra(Bhindi) or French beans instead of bittergourd.