Preheat the grill to medium-high heat (350°F).
In a small bowl, whisk together the olive oil, sesame oil, ginger, soy sauce, garlic powder, rice vinegar, and half of the sesame seeds.
Pour half of the marinade over the tuna steaks, reserving the other half for later. Cover and refrigerate for at least 20 minutes (or up to overnight) while the grill heats.
When the grill is scorching hot, remove the tuna steaks from the marinade and discard the excess marinade.
Coat the tuna steaks with 1 tablespoon of sesame seeds (or more if needed) to create a crust.
Place the tuna steaks on the grill and cook for 2-3 minutes per side, drizzling a bit of the reserved sauce on top while grilling.
For medium-rare tuna, cook until the internal temperature reaches 135°F. The FDA recommends cooking tuna to 145°F.
Remove from the grill and top with the remaining sauce, green onions, and additional sesame seeds.
Serve immediately and enjoy!