Pat the ahi tuna steak completely dry with paper towels and season both sides with sea salt and black pepper.
Combine the black and white sesame seeds on a small plate and press the tuna steak firmly into the seeds to create an even crust on all sides.
In a small bowl, whisk together the tamari, lime juice, and freshly grated ginger to create the zesty dressing.
Heat the toasted sesame oil in a cast-iron skillet over medium-high heat until just shimmering.
Place the tuna in the hot skillet and sear for approximately 45 to 60 seconds per side for a rare center, then remove from heat.
In a large bowl, toss the arugula, sliced cucumber, sliced radishes, and shelled edamame with half of the prepared dressing.
Slice the seared tuna into thin strips against the grain.
Plate the salad, top with the sliced tuna and avocado, and drizzle with the remaining dressing before serving.