1. In a mixing bowl, whisk the shallots with the pepper, mustard, mirin, and soy sauce. Gradually whisk in the sesame and vegetable oils.
2. In a large bowl, pour all but 1/4 cup of the dressing over the salad and toss lightly. Mound the salad high in the center of 8 serving plates. Garnish each with 4 slices of sashimi and spoon the remaining dressing alongside the tuna. Serve immediately.