Cook the melon / ash gourd, curry leaves and green chillies in about ½ cup water till it is soft but not mushy.
Rinse and cook the beans in a pressure cooker, with roughly the same amount of water for 2-3 whistles to be soft but not mushy. You can cook it on the stove top as well.
Drain the water from the beans, and keep it aside. Add the cooked cowpeas to the cooked melon. Add more of the cooked beans water if you need. Remember that we are going to add coconut milk to this, so you don't want the curry to be too thin.
Add salt and coconut milk and let it simmer for about 5 mins. Turn off the heat.
Drizzle some coconut oil over it while the olan is still hot, so as to release the wonderful aroma. Close with a lid to capture the aroma into the curry and let it sit till ready to serve.
Notes
If you are using homemade coconut milk, cook the melon in the second pressed thin coconut milk and add the thick first pressed coconut milk towards the end.I don't mind using the water from the cooked beans if needed but some are particular about the color of the olan to be white. If you want it to be white then don't use the water from the cooked beans as it may be pink or muddy white depending on the beans used. I love the reddish pink hue in Olan and that's a personal choice/.Once you add the coconut milk, especially the thicker milk when using homemade coconut milk, do not boil the curry. Let it simmer for just a few minutes for the flavors to get in. Otherwise the fat may separate and that is not gonna be good.