Add little more oil in that pan & add the red chilli & nigella seeds.
When the red chili turns brown add the chopped bittergourds and potatoes.(You can also boil them for 7-8 minutes if you like it to taste less bitter).
Add little salt & turmeric powder.
Fry them covered in low flame for 10 min. Stir in between.
Now remove the lid & fry in medium flame for few more minutes.
Add all the spices, the green chilies & also the fried prawns. Mix them well.
Now add little water. It will be a dry curry.
It tastes great with steamed rice in lunch time.
Check out other popular Prawn Recipes