Sweet and Sour Seared Tuna

Heat a generous amount of olive oil in a thick-walled cooking vessel over an intermediate heat setting, around 300°F (medium heat). Introduce the onion into the skillet and gently stir it until it has softened and turned a pale, translucent hue, approximately 5 minutes. Decrease the heat to a lower setting and continue cooking and stirring until the onion has become extremely tender and has developed a deep, rich brown color, taking around 15 to 20 minutes longer.

Remove the onion from the skillet.

Place the tuna steaks in the same cooking vessel over a high heat setting, around 400°F (medium-high heat). Cook the tuna steaks until they have turned a golden brown color, approximately 2 minutes per side. Transfer the tuna steaks to a serving tray and keep them warm.

Combine the vinegar and sugar in the same cooking vessel, stirring until the sugar has dissolved. Reintroduce the onions to the pan and simmer until the liquid has reduced and thickened slightly, taking around 5 minutes. Return the tuna steaks to the pan and cook for an additional 3 to 5 minutes.

Place the tuna on the warm serving tray and top it with the onion mixture, adding a sweet and sour sauce.