Use a few paper towels and pat-down the tuna. Then use a knife and cut into small bite-sized cubes (450 grams or 1 pound's worth).
Cut a ¼ of an onion into very thin slices. Measure out ¼ cup's worth.
Cut a stalk of green onion into very thin slices. Measure out ⅓ cup's worth.
Grate a small piece of ginger: Measure out ½ teaspoon(!) worth.
Take out a small mixing bowl and make the sauce: Mix Soy Sauce (1.5 Tablespoons), Sesame Oil (1 Tablespoon), Salt (¾ teaspoon!), Grated Ginger (½ teaspoon!)