This dish is particularly good when you use fresh, earthy mustard seeds. I prefer dark seeds for the way they look in the finished recipe, but friends have tried this with yellow mustard seeds too.
Combine all of the marinade ingredients, add the shrimp, and allow the shrimp to absorb the marinade for at least an hour. It's fine to leave the shrimp longer, but store it in the refrigerator if you do so.
After the shrimp have been in the marinade a sufficient time, heat a large 12-inch cast-iron pan (or equivalent) with 2-3 tablespoons vegetable oil. When the oil is hot, add the mustard seeds and cover the pan. After several moments the seeds will begin to pop, like popcorn.
When the popping has stopped, remove the lid and add the onions. Stir the onions into the popped mustard seeds. Reduce the heat slightly, and cover the pan until the onions are translucent (2-3 minutes).
When the onions are done, remove the lid and add the shrimp along with all the marinade liquid. Saute until the shrimp are cooked to an orang-pink color. Reduce the heat further and cook until the liquid thickens a bit.
Serve hot over a bed of basmati rice.