Beat the yogurt till smooth along with a pinch of salt and a pinch of sugar. Add water if thick, it should have a consistency of tomato sauce.
Heat the oil in small saucepan and add the mustard seeds. Wait for it to sputter (less than a minute).
Add the cumin seeds, red chili, and curry leaves to the oil and cook until cumin seeds turn a shade darker, less than a minute. Be careful as the curry leaves will release water and cause the oil to sputter.
In a bowl combine the mango, yogurt, and half the tempering. Adjust salt to taste.
Garnish with rest of the tempering and place in the refrigerator till ready to serve. Can be made in advance, serve cold or at room temperature.