Heat oil in a large dutch oven over medium heat. Add onions and 1 tsp of the salt. Cook until very soft and nicely golden, about 20 minutes, reducing heat if onions start to brown before they’re soft.
Add garlic, cayenne, cumin and turmeric and cook 1-2 minutes, until garlic is soft and spices are fragrant. Add the vinegar and mustard and cook a minute more, stirring frequently. Add the lamb and the coconut milk.
At this point, you can simmer on the stove for 1 to 1 1/2 hours, until lamb is tender, or transfer to a pressure cooker, stir in the sauce, and cook on high pressure for 30 minutes and let the steam release naturally. Top with cilantro.
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Notes
Bone-in lamb meat will give you more flavor than boneless, so consider using it if you have a good butcher.
This recipe yields a wonderful large batch of curry - handy if you want to feed guests and have leftovers for the freezer. You can also halve the recipe.
It gets better with age. Let it rest overnight or for a couple of days, and you'll notice the flavors develop further.
The vinegar and mustard are there to cut through the creamy richness of the sauce.