Whisk the tamari, rice vinegar, lime juice, grated ginger, and minced garlic in a small bowl to create the zesty dressing.
Pat the tuna steak dry with a paper towel and season both sides with sea salt and black pepper.
Spread the sesame seeds on a small plate and press the tuna steak into the seeds to create an even crust on both sides.
Heat a cast-iron skillet over medium-high heat with the toasted sesame oil until the pan is shimmering.
Sear the tuna for 1 to 2 minutes per side for a rare-to-medium-rare center, then remove from the pan and let rest for 3 minutes.
In the same skillet, quickly flash-sauté the bok choy and sugar snap peas for 2 minutes until bright green but still crunchy.
Slice the tuna against the grain into thin strips.
Arrange the sautéed greens on a plate, top with the sliced tuna, radishes, and avocado, then drizzle with the prepared dressing.