Add the sliced chili peppers to a large bowl.
Lightly toast the mustard seeds, coriander seeds and cumin seeds in a dry pan over medium heat for 1-2 minutes, or until they become fragrant.
Grind the seeds into coarse powder in a spice grinder, or use a mortar and pestle. Combine the ground seed mixture with the turmeric, garlic powder and onion powder (or asafoetida, if using).
Add the seasoning mixture to a small pot along with the vegetable oil, lemon juice, vinegar (if using) and salt. Gently warm on medium heat just a minute or two (do not boil), then pour the liquid over the sliced peppers.
Cover and refrigerate at least a day to allow the flavors to mingle and develop. It can be eaten right away, if desired.
