The fish in this dish is basically raw, so it's important to buy it from a dependable source, such as Fresh Mart (in the basement of Sogo, Causeway Bay), Apita (in Taikoo Shing), City'super (various locations) or Great (in Pacific Place). Choose a piece that's about 6cm wide and 2cm thick. I like to serve this as a starter, along with Japanese pickled cucumber and wakami seaweed.
250 grams sushi-grade tuna fillet
About 15ml cooking oil
2 tsp sesame seeds (preferably a mixture of black and white)
2 spring onions, white and pale green parts only, cut into thin shreds about 4cm in length
For the dipping sauce
25ml fresh lemon juice
50ml soy sauce
100ml rice vinegar
Finely grated zest of 1/4 a lemon
Mix together the sauce ingredients, taste for seasonings and correct if needed. This can be made several hours in advance, then refrigerated until needed.
Put sesame seeds in a small, unoiled pan and set over a low-medium flame.
Stir constantly until the sesame seeds are fragrant and lightly toasted; watch carefully because they burn easily. Pour them onto a small dish and leave to cool.
Set a skillet (preferably cast iron) over a medium-high flame and get it very hot.
Brush the tuna on all sides with the cooking oil, then place it in the skillet and sear quickly on all sides, just to lightly brown the exterior - it should take no more than 15 seconds per side.
Cool briefly, then use a sharp knife to slice the tuna against the grain into 8mm-thick pieces.
Lay the pieces on two plates, then spoon some of the sauce over the fish.
Add some spring onions and sprinkle with toasted sesame seeds.
Serve with additional sauce in small bowls. (Any remaining sauce will keep in the fridge for about a week.)