Pat the tuna steak dry with a paper towel and season both sides with sea salt and black pepper.
Press the black sesame seeds firmly onto the surface of the tuna to create an even crust.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Sear the tuna for 60 to 90 seconds per side for a rare center, then remove from the pan and let it rest on a cutting board.
While the tuna rests, dice the cucumber and avocado into bite-sized cubes and thinly slice the radishes.
In a small glass jar, whisk together the coconut aminos, toasted sesame oil, and lime juice until emulsified.
Combine the cucumber, radishes, and avocado in a bowl and gently toss with the prepared dressing.
Slice the rested tuna against the grain into half-inch thick pieces and serve immediately over the chilled salad.