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.