Soak the raisins in warm water for 10 minutes, then drain well and set aside.

Pat the tuna steaks dry with paper towels and season both sides with the salt and black pepper.

Heat 2 tablespoons of the olive oil in a 30 cm / 12-inch skillet over medium-high heat until it shimmers. Sear the tuna in two batches of two steaks so they don't crowd or steam, 90 seconds per side, until deeply browned outside and the center reaches 52 C / 125 F on an instant-read thermometer. Transfer to a plate and tent loosely with foil.

Lower the heat to medium and add the remaining tablespoon of olive oil to the same skillet. Add the sliced onion and cook, stirring occasionally, for 8 to 10 minutes until soft and pale gold.

Pour in the white wine vinegar and sugar, scraping up the browned bits from the bottom of the pan. Simmer for 2 to 3 minutes until the liquid reduces to a light syrup.

Stir in the drained raisins and toasted pine nuts and simmer for 1 minute more.

Return the tuna and any accumulated juices to the pan. Spoon the sauce over the steaks and warm through for 1 to 2 minutes, just until heated.

Remove from heat, scatter with the torn mint leaves, and serve right away.