Bring 750ml / 1 1/4 pints (3 1/4 cups) water to a boil in a large, heavy-based pan.
Add the tomatoes, chillies, curry leaves, and half of the onions, season with salt and reduce the heat slightly. Continue to boil for about 5 minutes.
Add the garlic paste, ground cumin, coconut, turmeric, and spinach, then reduce the heat and simmer for a few minutes until the moisture has evaporated.
Heat the oil for the tempering in a large frying pan (skillet) over medium heat, add the remaining onions, the dried red chillies, and mustard seeds and, stir-fry for about 5-7 minutes, or until the onions are golden brown.
Pour over the tomatoes and cover immediately to retain the aroma.