Pat the ahi tuna steak dry with paper towels and season generously with salt and freshly ground black pepper on both sides.
Heat 1 tablespoon of olive oil in a skillet over medium-high heat until shimmering but not smoking.
Carefully place the seasoned tuna in the hot skillet and sear for 1-2 minutes per side for rare, or 2-3 minutes per side for medium-rare.
Remove the tuna from the skillet and place it on a cutting board. Let it rest for 5 minutes.
Meanwhile, in a large bowl, gently combine the mixed greens, diced mango, sliced avocado, cucumber, and red onion ribbons.
Make the vinaigrette by whisking together the remaining tablespoon of olive oil, fresh lime juice, a pinch of salt, pepper, and soy sauce if using.
Drizzle about two-thirds of the vinaigrette over the salad mixture and toss gently to coat.
Slice the rested tuna against the grain into thin, ¼-inch slices.
Arrange the salad on a serving platter or individual plates, then fan the tuna slices decoratively on top.
Drizzle the remaining vinaigrette over the tuna slices and garnish with chopped cilantro and sesame seeds if using.
Serve immediately while the tuna is still warm and the greens are crisp.
