Pat the tuna steak dry with paper towels and season both sides with sea salt and black pepper.
In a small bowl, whisk together the toasted sesame oil, tamari, lime juice, and grated fresh ginger to create the dressing.
Dice the avocado and thinly slice the English cucumber, then place them in a medium bowl.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Sear the tuna for 1-2 minutes per side for a rare center, or until the desired level of doneness is reached.
Remove the tuna from the pan, let it rest for 2 minutes, then slice it into half-inch thick pieces.
Toss the cucumber and avocado with the prepared dressing and arrange on a plate.
Top the salad with the sliced tuna and garnish with black sesame seeds.