Slice the chillies in half lengthways, remove most of the seeds and then thinly slice.
Heat the olive oil in a heavy-based saucepan over medium heat and add the cumin seeds, mustard seeds, turmeric and ginger. When the mustard seeds begin to pop, add the garlic and chilli. Stir a few times, then add the chopped tomatoes, sugar and vinegar.
Bring to the boil, then reduce the heat