Heat oil in a pan over medium heat. When the oil is hot enough, add mustard seeds, urad dal and chana dal. Wait till the mustard seeds pop up and the lentils turn light brown.
Now add chopped onions and green chillies. Sauté until the onions become soft.
To this add the ridge gourd pieces, salt and turmeric powder. Sauté for a minute.
Cover with lid and cook for 2 to 3 minutes over medium heat.
Now add red chilli powder and curry leaves.
Stir well until well combined, cover with lid and cook again for 2 minutes over medium heat.
At this stage add milk and cook over medium heat till the vegetable is completely cooked and you get the desired consistency. Adjust salt if required.
Turn off the heat and garnish with finely chopped coriander.
Serve this luscious green Ridge Gourd Curry with rice, roti or paratha.