Make dipping sauce. In a medium bowl whisk together soy sauce, rice vinegar, and honey until honey dissolves. Whisk in sesame oil, ginger, and garlic until emulsified. Leave at room temperature or refrigerate if desired. (recipe note #1)
Preheat a gas grill (or charcoal grill) to high. (recipe note #2)
Rinse tuna steaks and pat dry with paper towels. Rub all sides of tuna with olive oil and season with salt and pepper. Grill tuna 2 minutes per side, until seared on the outside and rare in the center. (If you prefer a cooked center, cook an additional ~2 minutes per side.) Transfer tuna to cutting board. With a thin, sharp knife, thinly slice tuna across the grain (¼ to ½-inch thick slices). Sprinkle with sesame seeds and sliced green onion. Serve with dipping sauce.
If you prefer marinated fish, double the dipping sauce recipe and marinate the rinsed and dried tuna steaks for 20 to 30 minutes in ½ of the sauce before grilling. Serve the remaining sauce alongside for dipping.
As an optional step to prevent sticking, brush grill grates with a high heat neutral or vegetable oil just before you add steaks to the grill.
If desired, divide dipping sauce among 4 small bowls for serving.