Make the dressing by mixing together all the ingredients. Meanwhile, cook the noodles according to the instructions on the packet and set aside.
Press the sesame seeds into both sides of the tuna. Heat a large, heavy-based frying pan and dry-fry the tuna for 1—2 minutes on each side, depending on the thickness, until it is just pink in the middle.
Slice the tuna. Spoon the dressing over the hot noodles and put the sliced tuna on top. Garnish with coriander leaves and serve immediately.