Peel the onion and cut it into rings.
Peel the ginger and chop it in half.
Remove the stems from the coriander.
Peel the garlic.
Put the onion, ginger, coriander and garlic in the food processor and add the sushi vinegar, the soy sauce and the olive oil and sesame oil and mix.
Turn off the food processor when the ingredients are mixed into a dressing.
Place the tuna slices in a large container and pour some of the dressing over them.
Turn the tuna over and again pour some of the dressing over the tuna.
Make sure to leave some dressing, so that the lettuce can be made with it.
Cover the dish with foil and put it in the refrigerator for at least 5 hours, so that the dressing can absorb well into the fish.