Cut the chalkumra so that semi circle shapes form. Sprinkle the chalkumra with 1/2 teaspoon or a pinch of turmeric powder and some salt. Heat 2-3 tablespoon oil in a pan, and fry the chalkumra pieces until they turn red. Set aside.
Prick open the coconut and take out the coconut flesh using a coconut grater. Use half of the grated coconut to make a paste. The paste can be made by grinding the grated coconut on sheel pata or a blender using very little water.
The other half of the grated coconut is going to be used for making coconut milk. Pour 3 cups of boiling hot water and allow the grated coconut to soak well. Pour the mixture on a sieve and allow the milk to filter out. Use a spoon to press the grated coconut against the milk. It can also be squeezed to take out as much as milk as possible.
On the same pan, add 1/2 cup of oil and fry the sliced onions on high heat until they turn golden.
Add the shrimps, ginger, garlic and coconut paste. Sprinkle 1/2 teaspoon of turmeric powder and red chilli powder and stir well for 2-3 minutes.
Add the green chilies and stir again.
Sprinkle salt and pour in the coconut milk. When the milk boils, add the fried chalkumra pieces one at a time. When the water dries up a bit, turn the heat down and cover the pan with its lid for 10 minutes. At end of 10 minutes, the chalkumra will be completely cooked, with the oil floating on top.