Combine mustard, cumin, turmeric, cayenne, salt and vinegar in a cup and mix well. Set aside.
Put oil in large frying pan over medium-high heat. When the oil is hot, add the onion and cook until it's medium brown. Add the garlic and cook another 30 seconds. Add the spice paste and cook another minute. Add meat and stir-fry for about 3 minutes more.
Add the coconut milk and water, cover & bring to a boil. Lower heat to a simmer for 60-70 minutes.
** Optional .. Depending on the thickness and amount of sauce you prefer, the liquid can be reduced at this point. (I like to reduce the liquid until it's about 1/2 the amount the original recipe calls for).
Uncover the pan, bring the liquid to a boil and simmer until the sauce is 1/2 the amount you started with. (About 10-15 minute)