Tuna is characterised by its oil-rich flavour and flaky texture. Steaming is an efficient method of cooking as it usually creates less mess and retains more food nutrients than boiling or roasting. Steaming fish or vegetables also helps retain flavour.
Purchase your fresh steak from a fishmonger or supermarket. Find a steak that has been trimmed neatly with firm flesh and a fresh, meaty smell. Avoid steaks that are discoloured near the bone or those with brown patches. Avoid choosing bluefin tuna, too, if you are concerned about the environment: bluefin is an endangered species. (Not to mention, it's rather expensive, used mainly in sushi and gives off a fishy smell and taste when cooked.)
Refrain from washing your tuna steak before cooking. The high oil content keeps tuna fresh. Use a kitchen towl, rather to dab off any excess moisture.
Fill a steamer with 1 to 1-1/2 inches of water and bring to a boil. Put the tuna steaks into the steamer and cover with a lid.
Steam for 6 to 8 minutes.
Combine 1/2 cup of soy sauce, 1/2 cup of sherry and 1/2 cup of vegetable oil in a saucepan over medium-high heat.
Finely chop 6 or 7 spring onions (trim and discard about an inch from the top and bottom of each). Crush 3 cloves of garlic with a garlic press. Add the spring onions and garlic to the saucepan.
Place 2 tbsp of minced root ginger, 1 tsp of salt and 1 tsp pepper into the saucepan. Bring the mixture to a boil.
Remove the tuna steaks from the steamer and place in a large serving dish. Pour the sauce from the pan over the tuna. Serve immediately.
Check the tuna's readiness by rubbing its edge with a fork. If the fish flakes easily, your steak is ready to be eaten.