For the vinaigrette: In a small bowl, combine the rice vinegar, olive oil, sesame oil, mustard, sugar and some salt and pepper. Whisk until the sugar has dissolved and the vinaigrette is emulsified. Cover and store in the refrigerator if not using immediately. (Makes about 5 tablespoons.)
For the tartare: In a medium bowl, gently toss together the tuna, cucumbers, scallions, shallots, sesame oil, black sesame seeds, roasted sesame seeds, 2 perilla leaves and 2 tablespoons of the vinaigrette. Season with salt.
Divide the tartare between two bowls or plates and sprinkle with additional roasted and black sesame seeds and the julienned perilla leaf.
Add more of the Spicy Korean Mustard Vinaigrette, if you like. And feel free to increase the amount of mustard too, if you prefer a bit more kick.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.