Heat a little oil in a frying pan, fry the eggplant cubes. Keep aside on kitchen absorbent towel.
Marinate the fish with salt, a pinch of turmeric and 1 tsp of mustard oil. (The mustard oil ensures the fish doesn't splutter naughtily when dipped in hot oil ? ) Heat 1 tbsp mustard oil in a frying pan and lightly fry the fish (Please don't over-fry as the fish can get stiff). Keep aside.
Add the remaining oil to the same pan, when smoking hot, throw in the kalonji, allow the seeds to splutter. Add the chopped onions. Fry till golden brown.
Now throw in the pumpkins and radish. Saute for a couple of minutes.
Sprinkle the turmeric powder, red chili powder and salt, add the green chillies, give it a hearty stir. Cover and cook over a medium flame for another 5-7 minutes.
Now add the ridgegourd, sheem and fried eggplant cubes. Splash a little water as and when needed if it is getting a bit too dry and threatening to stick to the bottom of the pan. Cook till the vegetables are tender.
Add the fried fish, mustard paste, toss gently. Adjust seasonings and cook for a final 4-5 odd minutes over a low flame.
Finish with coriander leaves. Serve hot with rice.