Blend all the ingredients for the spice paste with 1 cup of water.
Line a fine mesh sieve with a muslin cloth, transfer the ground paste to the cloth, and extract all the coconut milk. This is the thick coconut milk extract.
Transfer all the leftovers to the blender, add 1.5 cups of water, blend again. Repeat the same procedure of extracting the coconut milk. This is the thin coconut milk extract.
Heat oil in a heavy bottomed pan, add chopped onions, sauté until it turns translucent. You can add the slit green chillies at this point too.
Add chopped tomatoes, sauté until it turns mushy.
Then add the thin coconut extract along with a pinch of salt, mix well, followed by the tamarind extract. Bring to a boil.
Add the bottle gourd cubes, and cook over medium heat until about 80 % done.
Then add the shrimps, cook another 4-5 minutes or until it is cooked through.
Add the thick coconut extract, swirl the pan gently. Simmer for another minute and switch off the heat. Do not cook it for a long time or else the coconut milk will curdle. Check for seasonings and adjust with salt if required. Serve hot with rice.