Place the fennel, mustard seeds and cardamom seeds in a spice grinder, or use a mortar and pestle to grind to the consistency of finely ground black pepper; transfer this spice blend to a large bowl.
Add the garlic, chili peppers, salt and scallops to the bowl and stir to mix, making sure to coat the scallops well.
Cover and refrigerate scallops until ready to cook; because the mixture is not acidic, you can marinate overnight, if desired.
In a large skillet, using high heat, heat the oil.
When the oil appears to shimmer, add the coated scallops in a single layer; do this in batches if skillet is not large enough.
Cook without moving until well-seared, about 2 minutes; turn and cook 1 minute more.
Remove to a serving platter and repeat, over medium heat, with additional scallops, if necessary.
Add coconut milk to skillet — it should bubble immediately — and cook, scraping up the brown bits on the bottom of the pan, until thickened; about 30 seconds to 1 minute.
Pour sauce over scallops; strain through a sieve, if desired, for a more aesthetically pleasing appearance.
Serve warm, sprinkled with cilantro.
