Place tuna in a shallow bowl.
Mix soy sauce and mirin in a small bowl. Pour over tuna. After 5 minutes, flip tuna and allow it to marinate for another 5 minutes.
Mix all wasabi mayo ingredients together thoroughly in a small bowl. Set aside.
Heat a cast iron pan on the stove over medium high heat. Sear tuna on all sides for 1 to 2 minutes on each side to form a slight crust.
Remove and slice tuna into ¼ inch thick slices. Serve immediately with wasabi mayo.