Heat oil in a saucepan and add the curry leaves, mustard seeds and onion, and cook for about thirty seconds to temper the spices (they will just begin to sputter).

Add the garlic and ginger, turmeric, asafoetida, coriander, red pepper, crushed tomatoes, tomato sauce, vegetable broth, seasoning to taste with alt and black pepper.

Bring mixture to a boil then reduce heat and simmer, covered, for about 15 minutes.

Use a hand blender to puree this in the pan, or alternatively, you may choose to let it cool somewhat then puree in a food processor or blender.

Return soup to pan and whisk in the yogurt.

Taste for acidity (if your yogurt is tart you might want to sweeten it a bit with a pinch of sugar to taste). recheck salt and pepper, then warm briefly.

Serve hot drizzled with sriracha sauce (if desired) and grilled pieces or nan bread or roti.