Heat a skillet over medium heat. Swirl in the vegetable oil and sear all sides of the tuna about 30 seconds each.
Once the tuna is seared on all sides, transfer it onto a plate and drizzle on the soy sauce while still hot. Wrap it with plastic wrap and chill in the refrigerator for a while to let the taste penetrate.
Rinse the mizuna with water, cut into about 5 cm lengths and drain the water well. Cut the tomatoes into bite-sized piece.
Combine the ingredients for the dressing. Add the water gradually toward the end to avoid it from separating!
Add half the amount of dressing onto the mizuna and tomato, and toss them evenly.
Once the tuna is cool enough, slice into the desired thickness.
Spread the mizuna and tomato in a serving bowl and top them with the sliced tuna. Pour in the remaining half of the dressing and sprinkle with green onion and nori seaweed if desired and it is done.
Story Behind this Recipe
I had a craving for mizuna, so I made this salad with the tuna that was on sale.
If you are using grain vinegar, add 1 tablespoon of it first, then check the taste and adjust the amount.