Heat three tablespoons of the oil over medium heat in a large nonstick frying pan.
Add the onions and cook for seven minutes, stirring often, until they are soft and lightly browned. If they start to brown too much, reduce the heat.
Add the vinegar, sugar, salt and pepper and stir. Add the juice, turn the heat up to high and cook for three minutes, stirring often, or until the liquid is reduced by half.
Scoop into a bowl. The onions may be served warm or cold They can be made up to three days in advance.
Heat the pan over high heat and add the remaining 1/2 tablespoon of oil. Add the tuna steaks and cook for one to one and a half minutes, per side, for rare, longer if you like your tuna more cooked.
Remove from the pan to a cutting board and slice, and then place onto a platter. Add the pickled onions on top.
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