Pickled onion mackerel

1 large mackerel
1 onion
~1/2 cup tarragon vinegar
Olive Oil
Salt

Slice the onion and soak for about 30 minutes in the vinegar.
Cut the mackerel in half lengthways (turn on its back and cut to one
side of the spine).

Arrange the mackerel skin-side up, on top of the onion, in an
ovenproof dish. Pour the vinegar into the dish. Sprinkle a little salt
over the fish and a little olive oil on the skin.

Cook under a medium grill until the skin is nicely browned and the
flesh is cooked throughout.