Drain the water out of the Chana Dal and set aside, discarding the water.
In a medium sized pressure cooker, heat the vegetable oil on medium heat.
Add the black mustard seeds and asafetida and fry for a few seconds until fragrant.
Immediately add the diced opo squash, Chana Dal, turmeric, ground cumin-coriander powder, jaggery (or dark brown sugar), salt, chili powder (if using), chopped tomatoes, and water. Combine thoroughly.
Lock the lid in place for the pressure cooker and turn the heat to medium-high. Once the steam begins to hiss out of the whistle, lower the heat to medium until you hear 2 to 3 more whistles (note that depending on your pressure cooker this may vary slightly). If your pressure cooker does not whistle, then when the steam begins to hiss out of the cooker, reduce the heat to low-medium just enough to maintain a very weak whistle. Cook for an additional 2-3 minutes.
Remove the pressure cooker from the heat and let it sit for about 15 minutes (the curry will continue to cook inside the pressure cooker). Carefully release the steam and open the lid of the pressure cooker.
Place the pressure cooker (lid open) on medium heat once again and carefully mix the curry.
Add salt to taste (if you need more sweetness, add more jaggery) and burn off any extra liquid if necessary (note that the curry should be pretty thick and not runny).
Turn off the heat, add freshly chopped cilantro, and serve hot with a side of your favorite tortilla (Naan, Roti, etc.) or rice!