Gather all your ingredients and prepare the tomatoes.
Stir the yogurt briefly to smooth out and add salt and pepper.
In a small pan, heat oil over medium-high heat. Add mustard seeds and curry leaves and cook for 60 seconds or more until the seeds sizzle.
Add onion, chili, cumin, grated ginger, and asafetida, if using. Stir for 3 to 8 minutes until the mixture turns a darker shade, and you have a cooked paste. Add water if the mixture is sticking to the pan. Note: If your tomatoes are firm they will take longer to cook down.
Cool and mix the masala into the yogurt. Chill for approximately and hour, and then taste for seasoning before serving.
Curry leaves: Sauté 4 or 5 small curry leaves in a teaspoon of oil.
Cilantro: Coarsely chop a couple of tablespoons of cilantro.
If you prefer, you can keep the tomatoes raw for a little more texture and freshness. Just cook the rest of the ingredients for a couple of minutes, cool and mix into the yogurt.
Instead of chopped tomatoes, you can use 1 cup of tomato puree, for more tomato flavor. There is no need to cook the purée but cook the rest of the ingredients as instructed in the recipe above.
If you are going with either of the options above for tomato raita, you will still want to mix the ingredients together and chill at least an hour before serving, so that the flavors can mellow together.
If you don't eat onion, skip it. Ideally, use the optional pinch of asafetida, which will add a similar umami taste that you get from an onion.