1kg piece of tuna loin fillet (cut from the thick end of the fillet, about 18cm long)
Sunflower oil, to brush
1/2 cup (125ml) dark soy sauce
1/3 cup (80ml) balsamic vinegar
1/4 tsp chilli powder
Coriander, to serve
Lime wedges, to serve
Cut the tuna fillet lengthways into 3 long, narrow pieces that measure about 4cm across. If you wish to give them a nice cylindrical shape, wrap each piece tightly in plastic wrap so that it looks like a fat sausage, then chill for at least 3 hours.
Heat a large frypan over high heat. Unwrap tuna if necessary, brush with oil and sprinkle liberally with salt and pepper. Sear tuna for 30-45 seconds on each face until coloured all over and cooked to a depth of 5mm – the centre should remain raw. Remove from pan and set aside.
Return pan to medium-high heat, add soy, balsamic and chilli and simmer for 3-4 minutes until well reduced and syrupy. Take pan off heat, return tuna to pan and turn in glaze until well coated. Set aside to cool for 15 minutes, turning tuna now and then so it takes on more glaze.
Just before serving, cut the tuna into 5mm-thick slices and arrange on a plate. Serve garnished with coriander and lime.