Mackerel is a delicious and a sustainable fish and very reasonably priced but not as popular as it should be.
For the years I ran the restaurant I tried to persuade people to eat it by putting it in the menu but it was decidedly a non-runner.
This I firmly believe is that it is a bony fish and people are afraid of the bones. There are various ways to get rid of the bones, my wife has an ingenious method of pressing on its back to remove them, but I have found the only fool proof way to get rid of all the bones is to cut them out, not at all difficult with a sharp knife.
If you lay a fillet on a board you will see that the principal bones on the fillet run in a straight line down the middle, these are so numerous that I have found the only way to be sure all are removed is to cut them out.
Using your sharp knife cut at each side of the line of bones effectively removing a narrow band of flesh with the bones in.
Throw this away, or use with the head to make a fish stock.
Now check the edges of the fish with your fingertips and cut out or pull out any remaining bones there. Do this thoroughly as mackerel bones are very unpopular.
This divides each fillet into two boneless goujons, each fish into four.
Now you can cook the fish in different ways.