The key to good Tuna sashimi
is to start of with a good piece tuna fish, what you want to look for is a red pink color tuna sirloin (not dark red as that would indicate that the fish was not bled properly), the tuna has to be firm to the touch and not sticky. It should smell neutral.
Cut the tuna in to rectangular blocks about 3 cm x 5 cm
Chop up the coriander finely.
Cover the tuna in the coriander
Sear the tuna in a none stick pan with NO oil about 15-20 seconds each side.
Let tuna rest on a plate for 5 minutes, then cut it vertically down in to even slices.