This recipe is ultra-simple: just fish, fennel seeds, garlic and lemon. That's it. That's what we love about fish - it usually does best in very simple preparations, especially on the grill or under the broiler.
A note on mackerel: according to Blue Ocean Institute's Guide to Ocean-Friendly Seafood, mackerel has really made a comeback and is in plentiful supply. But! (Wouldn't you know it...) there's still a catch. Mackerel is high in mercury or PCBs, so if you're feeding this to children or pregnant women, we'd recommend skipping the mackerel in favor of another fish that is low in mercury.