Sweet and sour tuna with onions (Tonno in agrodolce)

1. Peel the red onions and slice them into thin half-moons.

2. In a frying pan, cook the onions in 2 tablespoons of olive oil over medium heat for 10-12 minutes until soft and translucent; then sprinkle with the sugar, pour in the wine vinegar and bubble for 3-4 minutes until the liquid turns syrupy.

3. Pat the tuna steaks dry, season with salt and pepper and dust both sides lightly with flour.

4. Heat the remaining oil in another pan and brown the tuna on both sides. Lower the heat if necessary and cook until the centre reaches at least 75 °C for 30 seconds; the time depends on the thickness of the slices.

5. Place the tuna on a platter, pile the sweet and sour onions on top with their juices and scatter with roughly torn mint leaves.

6. Serve freshly cooked and warm. If preparing the day before, cool rapidly in a small, shallow container and refrigerate within 2 hours at most. Serve cold the next day, removing it just before serving. Do not leave it at room temperature for a prolonged period.

Tip: Cook the fish only as long as necessary and check doneness with a food thermometer. The onion dressing also keeps fully cooked slices moist.