In a small bowl, mix together soy sauce, mirin and sesame oil.
Place tuna steaks in a container with a lid and pour marinade over them. Let marinate in the fridge for 30 minutes.
Lightly pat dry tuna steaks with a paper towel, then pat on sesame seeds to coat.
Heat up olive oil in a cast iron skillet over high heat. Sear tuna steaks for 30 seconds on each side or until desired doneness.
Cut into thin slices. Serve with a wedge of lemon and your favorite grilled vegetables or sides. Enjoy!