In a small bowl, mix together the olive oil, sesame oil, soy sauce, salt, and black pepper to create a marinade.
Place the tuna steaks in a shallow dish and pour the marinade over them, ensuring they are well coated. Let them marinate for about 5 minutes.
Heat a non-stick skillet or grill pan over medium-high heat for 2-3 minutes until hot.
Remove the tuna from the marinade, allowing any excess to drip off. Place the tuna steaks in the hot skillet or grill pan.
Sear the tuna for about 1-2 minutes on each side, depending on how rare you like the center. For a medium-rare steak, you can sear 2 minutes per side. The outside should be nicely charred.
Once cooked to your liking, remove the tuna steaks from the pan and let them rest for a minute.
Slice the steaks against the grain into thick pieces and sprinkle with sesame seeds if desired.
Serve with sliced avocado and lemon wedges on the side.