Oh, and I have a hard time finding mackerel in my local grocery store, so I just bought some at Amazon. $40 for a dozen cans; since each can provides about 4 servings of curry, that's a pretty inexpensive meal.
Take 2-3 onions, dice, saute in 2T veg. oil with 1 t. mustard seed, 1 t. cumin seed. Cook until translucent golden. Add 1-2 T red chili powder, cook for a minute or so, until it starts to make you cough. Add 1 t. salt, and 1 t. Jaffna curry powder if you have it. (You can buy it online.) Add about 1/3 c. ketchup, and 1 T of worcestershire sauce. 1 T of fresh lime juice if you have it. Open one 15 oz. can of mackerel, drain liquid into pan. Stir that all together 'til well blended.
At that point, I'd usually let it simmer for 5 minutes or so, then check the flavors, adding more chili, salt, lime if needed. If it's too spicy, or just if you like the richness and sweetness, add 1/2 - 1 can of coconut milk. That's totally optional, though if you're not adding it, you might want to put in the liquid from a second can of mackerel. Or you can always use just a can's worth of water instead.
When you have a good sauce that you like the flavor of, add 2 cans mackerel. Simmer gently, stirring as little as possibly, until sauce is reduced to a pleasant gravy thickness. Usually about 15 minutes on medium-high heat.
We sometimes add 4 hard-boiled eggs, sliced in half, to the dish at the very end. Serve with uppuma or rice or bread. Enjoy!