Make the dressing: In a small bowl whisk the grated ginger, minced garlic, chopped red chilli, sesame oil, peanut oil, honey, soy sauce, and chilli powder or togarashi until combined.
Cut the tuna into thick, medium dice so the cubes hold their shape.
Toss the tuna gently with most of the dressing in a mixing bowl until evenly coated; reserve a little dressing for finishing.
Prepare the vegetables: dice the avocado and cucumber, and thinly slice the radish if using.
Assemble the tartare: Spoon diced avocado into the base of serving bowls, top with the diced cucumber, then mound the marinated tuna on top.
Finish and serve: Drizzle the reserved dressing over the tuna, sprinkle with toasted black and white sesame seeds, add radish slices if using, and serve with lime wedges.