Cut the bottle gourd into 1/2 inch pieces and potatoes in 1/4 inch pieces.
To a heavy bottom pot, add oil. Let it heat on medium flame.
Once the oil is hot, add mustard seeds, cumin seeds, and hing. Let the mustard seeds and cumin seeds crackle.
Add tomatoes, ginger garlic paste, and cilantro stems.
Add all the spices, turmeric, red chili powder, cumin-coriander powder and salt.
Cook until the tomatoes are mushy.
Now add the chopped bottle gourd and potatoes.
Add water, and let it simmer on medium flame. Add sugar to balance the tartness of the tomatoes, but this is completely optional. I only add it if the tomatoes are very sour.
Once the bottle gourd and potatoes are cooked, turn off the stove and garnish with lots of fresh cilantro.
Serve it with rice and. rotis.