To make the teriyaki sauce:
Heat a saucepan on medium, add the mirin, tamari, honey, sesame oil, garlic, and ginger, bring to a boil. Reduce the heat to low and simmer for 5 minutes. Strain the sauce to remove the garlic and ginger.
To make the pepper-crusted tuna steak:
Coat the tuna steaks evenly on all sides with freshly-ground black pepper. In a frying pan, heat the canola oil on high heat until smoking. Place the tuna steaks in the frying pan and baste with the teriyaki sauce. For medium-rare steak: cook for 1 minute, flip and cook for another minute.
Slice the tuna against the grain to sever the muscle, making tuna even more tender. Serve with a generous scoop of wasabi smashed potatoes. Drizzle the teriyaki sauce over both.