Heat oil in a skillet. To the hot oil, add mustard seeds and chilis. When the mustard seeds have popped, add in chilies, curry leaves, and asafetida if using.
Add tomatoes. Cook until the tomatoes are a little soft but still retain their shape.
Allow them to cool, and then mix with the yogurt right before serving.
If your tomatoes are over ripe and very sweet, add a little tang with the lemon juice.
If you only have Greek yogurt, add a little water to thin it down.
You can also try this with canned tomatoes if that's all you have access to.
This tastes like an Indian version of Rotel :)
All the heat in this recipe is from the Serrano, so use as much or as little as you can tolerate or enjoy.
This is a great make ahead salad as the flavors intensify overnight in the fridge. Eat some now, rest later.
You can also serve this as a dip with chips for something a little different.