Heat the oil in a large flat frying pan until just shimmering. Add the mustard seeds and cook until popping.
Add chilli flakes and salt and infuse briefly before turning the heat off and standing back.
Throw the curry leaves in from a distance and watch the mess...but such a yummy mess.
Store in an airtight container until use, but I suggest using within a week as it doesn't do well in the fridge and the leaves lose their crunch over a short time.
With curry, toasted sandwiches or waffles, soups, naan for starters, then let lose with anything else you like!