In a medium-size saucepan, combine the balsamic vinegar, honey and tamari. Place over high heat to bring up to a bubble and reduce until a thick syrup forms with a consistency like maple syrup straight from the fridge. It should take about 7-10 minutes.
While the balsamic reduction is cooking, preheat a large skillet over medium heat with 2 turns of the pan of EVOO, about 2 tablespoons. Season one side of the tuna steaks with the Chinese 5-spice powder, salt and freshly ground black pepper. Add steaks to the hot skillet, seasoned side down and cook for 2 minutes.
Season the other side then flip the tuna over and continue cooking for an additional 3-4 minutes. The steaks should be firm but have a little give with some pink at the center.
While the tuna is cooking combine the toasted sesame seeds and chives in a shallow dish. When the tuna is done, remove it from the skillet and press both sides in the sesame seed, chive mixture. Once they are all coated, slice each steak on the bias.
Place the shredded lettuce on a serving platter. Top the shredded lettuce with alternating slices of the tuna and avocado. Drizzle the entire dish with the balsamic reduction and serve.