Soak the ridgegourd slices in salt water for 15 odd minutes. Drain the water, squeeze out any excess water and keep aside.
Soak the black and yellow mustard seeds in warm water for 15-20 minutes. Drain the water, grind the mustard to a smooth paste along with 2-3 green chillies, a pinch of turmeric powder, a couple of rice grains and 2-3 tbsp of water.
Take the pumpkin and ridgegourd in a bowl. Add the grated coconut, mustard paste, salt, sugar and 3 tbsp mustard oil. Mix well.
Heat a frying pan, add the ridgegourd and pumpkin along with the marinade. Cook over a high flame for 4-5 minutes with frequent stirring.
Now turn the heat to low, let the vegetables cook patiently in their own juices. (The vegetables shall release a lot of water, so no need to add any water.)
Continue to cook with frequent stirring till the vegetables turn tender. (Care should be taken that the pumpkin and ridgegourd do not become too mushy.)
Adjust seasonings.
Finish with a generous sprinkle of grated coconut and a hearty drizzle of mustard oil.
Serve hot with steamed rice.