To make the wasabi smashed potatoes: Boil potatoes in salt water until fork tender, about 10 to 15 minutes. Strain and place in a big bowl. Add wasabi, soy milk, salt, and pepper and smash together. Don't over-mash.
To make the Teriyaki Sauce: Heat a saucepan on medium, add 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 garlic and ginger.
To make the Pepper-Crusted Tuna Steak: Coat tuna steaks evenly on all sides with freshly-cracked black pepper. In a frying pan, heat the canola oil on high heat until smoking. Place steaks in frying pan and baste with teriyaki sauce. For medium rare steak: cook for 1 minute, flip and cook for another minute.
Slice tuna against the grain to sever muscle, making tuna even more tender. Serve with a generous scoop of wasabi smashed potatoes. Drizzle teriyaki sauce over both.