Bonito encebollado : white tuna in dark onion sauce

Lately my life is one of being tied to my desk all day. When I do leave the house in the daytime, it´s to run short errands. But I can never resist going into the market, just to see what´s there.
The other day I wandered in, aimlessly, just because I´d been to the bank and I had a vague idea of buying some fruit.
And then I saw the freshest, most beautiful looking bonito (white tuna, albacore). I had to have it.
Rashly, without thinking, I asked the fishmonger to cut it into chunks, as for marmitako.
Once home, I was sorry, because it would have been heavenly as a big slice,marinated in soy sauce and mirin, and then lightly grilled. But it was too late for that.
I was put off the marmitako, in a fit of pique with myself, so I had to think of another way to do it.
Bonito encebollado came to mind. It´s one of those brilliant ideas, a very simple dish that only requires a bit of patience to do, and pays back a thousand times.
It´s also pretty good for doing ahead. You can make the onion sauce in the morning when it´s still cool, and only do the fish when you want to eat.

You chop a ridiculous amount of onions. For 500 grams of fish, I´d say about 4 or 5 big ones. Peel and slice thinly, or chop. Heat a big skillet or sautee pan with enough olive oil to cover the bottom. Add the onions, and let them cook slowly. When they´ve been there for 15 minutes, I like to add a heaped spoonful of brown sugar.
After 30 minutes or so, they should be caramelized and dark. Add 200 ml of oloroso sherry, or some other generous wine that´s not very sweet. If it is, then don´t add sugar before.

Leave to boil away, and when it´s a dark syrupy mess, add the salted tuna chunks, and grind black pepper over. When they´re white on one side, that´s in seconds, turn them over, leave for a minute, and then cover and turn the fire off. The residual heat will cook the fish, and it will be perfect in five minutes. It´s important not to leave it dry.

Serve with plain white rice.

Quantities for two hearty eaters (aka outright pigs) or three normal people.