Put the tomatoes, coconut, dal, turmeric and chilli powder in a blender or a food processor.
Season with salt and process to make a puree.
Pass through a sieve (strainer) into a bowl.
Put the coriander leaves in the blender.
Process to make a puree. Set aside.
Cook the onions in a pan of salted boiling water for 2-3 minutes, or until soft, then drain.
Heat the oil for the tempering in a heavy-based pan over medium heat.
Add the dried red chillies, mustard seeds, urad dal, cumin, curry leaves and asafoetida.
Stir-fry for about 1-2 minutes, or until dark brown.
Add the boiled onions and fry lightly.
Then stir in the tomato-coconut puree until it thickens.
Add the sugar, coriander puree and lime juice to taste.
Then cook for 1-2 minutes, or until heated through.
The chutney can be stored for up to 2 days in the refrigerator.