Heat the oil in a wok or deep skillet over medium heat, until very hot. Add the cumin, coriander, and mustard seeds, and cook for 10 seconds, until aromatic (keep a cover handy to stop any seeds from flying out). Add the onion and stir-fry for 5 minutes, until translucent. Add the ginger, garlic, and chili, and stir-fry for another 5 minutes.