Pat the tuna steak dry with paper towels and season both sides with sea salt, black pepper, and sesame seeds, pressing the seeds into the flesh.
Heat a non-stick skillet over medium-high heat and lightly coat with a small amount of oil spray if needed.
Place the tuna in the hot pan and sear for exactly 1 minute per side for a rare center, or longer if preferred.
Remove the tuna from the pan and let it rest for 2 minutes before slicing into 1/2-inch thick strips.
In a small bowl, whisk together the tamari, sesame oil, and rice vinegar to create the dressing.
Place the warm cooked brown rice in the center of a bowl.
Arrange the sliced tuna, avocado, cucumber, edamame, and thinly sliced radishes on top of the rice.
Drizzle the dressing over the entire bowl and garnish with thinly sliced green onions.