In a large saucepan, cook onions in oil over medium heat until nicely golden, about 20 minutes, reducing heat if onions start to brown before they're soft.
Meanwhile, whisk together the vinegar, mustard, spices and salt in a small bowl.
Add garlic to onions and cook 1 minute, then add vinegar mixture and cook a minute more, stirring frequently. Stir in coconut milk.
At this point, you can add the pork, cover and simmer on the stove for 1 to 1 1/2 hours, until pork is tender, or add the pork to a pressure cooker, stir in the sauce, and cook on high pressure for 30 minutes. Let the steam release naturally. Top with cilantro.
Video
Notes
Use water instead of coconut milk, and double (or triple) the cayenne if you want a more fiery version like what you'd have in India.
My Indian mother-in-law says to always choose bone-in meats for curries because the bone adds so much flavour. Plus, the meat nearest to it stays meltingly tender. Snip most of the meat away from the bone with kitchen scissors and cut it into chunks, then toss the meaty bones right in the curry to simmer, too.