Mackerel, how to get the good stuff?
Simple: go to your local Japanese or Korean grocer.  They will have your "good", oily, brown-fatted mackerel in the frozen section: look for or, better yet, ask for しおさば (shio-saba) in Japanese or godeung-eo in Korean.  This is invariably filleted, salted, and quite cheap.

This is most commonly served grilled, which is ridiculously easy: pop in the oven/grill for 15-20 minutes skin-side down on foil and serve.  My wife cooks this all the time, most recently earlier today for dinner!  Sample recipe in English here, but you can skip right over steps 1-6 if you're using pre-salted fish.

And for avoidance of doubt, frozen salted fish is not suitable to be eaten raw.  However, the friendly grocer can most likely tell you where he buys his fish, and in better Japanese shops may even have sushi-grade mackerel on hand.