Heat the oil in a saucepan on medium heat and fry the curry leaves, dried chillies and mustard seeds for 30 seconds.
Add the onions, and cook for 2 minutes, stirring a couple of times.
Add the tomatoes, sugar, salt and vinegar and bring to as simmer.
Then, lower the heat right down and cook for 20-30 minutes, until the chutney is thick and the tomatoes are very soft and have mostly lost their shape. If it gets too dry, add just a tiny amount of water, but you shouldn't need to. You could squash the tomatoes slightly, if you like. I cooked it for 20 minutes only.You'll have to stir the chutney a few times during this cooking period, especially at the end, when it's dry and will start catching at the bottom.
Check the seasoning and add more salt or even sugar or vinegar if you feel like you would prefer it saltier, sweeter or more tart.If adding more vinegar, cook it for an additional 5 minutes.
Cool and store in the fridge as mentioned in the post above.