In small saucepan, heat oil over low heat. Cook green onions, onion, red chili, salt and pepper, stirring occasionally, until onions are tender, about 20 minutes. Transfer to bowl.
Vinaigrette: In a blender, pulse together soy sauce, yuja preserve, lemon juice and mustard paste. With the motor running on low, slowly drizzle in olive and canola oils until emulsified.
Arrange tuna slices on 4 plates. Top each with some of the onion mixture, chives and tobiko. Drizzle each with some of the chili oil and vinaigrette. Arrange radish and watercress on each plate.
