Thickly slice the avocados lengthwise and coat with lime juice to prevent browning; set aside. In a small saucepan over medium-high heat, bring the vinegar, Tamari and honey to a low boil. Lower the heat and simmer until syrupy, 6-7 minutes. Set aside.
In a large skillet, heat the oil over high heat. Sprinkle the tuna with the five-spice powder and season with salt and pepper. Place the tuna in the skillet and cook, turning once, until just browned, 1-2 minutes on each side. Slice the tuna and top with the sesame seeds and chives.
Divide the lettuce among four plates and top with the tuna and avocado. Drizzle with the dressing.