Whisk vinegar (or lemon juice), canola oil, soy sauce, sesame oil, sugar and ginger in a small bowl.
Combine 3 tablespoons of the dressing with tuna, peas and scallions in a medium bowl.
Divide cabbage among 4 plates. Mound one-fourth of the tuna mixture (about 1/2 cup) in the center of each plate and garnish with radishes, cilantro and sesame seeds. Drizzle with the remaining dressing (about 2 tablespoons per salad) and season with pepper.