Prep ahi. Place the ahi fillet on a clean, dry surface. Lightly sprinkle all of the sides with sea salt, then either sprinkle or grind black pepper directly onto the top and bottom surfaces of the fillets. The pepper should be a thin, but generous layer.
Sear ahi. Coat a small sauté pan with grape seed oil and place it over high heat.Once the pan is very hot, add the Ahi and sear each side until it's only cooked through ⅛ to ¼ inch, 1 minute or less per side. (If you don't hear a sizzling sound when the tuna hits the pan, it's not hot enough -- wait for the sizzle!)
Slice ahi. Remove the seared tuna from the pan and place it on a clean plate. Once it has cooled completely, cut it into thin, evenly sized slices -- about ¼ inch each. Set aside. (You should have at least 14 slices.)
Prep mango and avocado. Cut the mango into halves. Then, slice each half into approximately 14 evenly sized slices, about ¼ inch each. (Here's How to Cut a Mango.)Cut the avocado into halves. Then, horizontally, slice the half into approximately 14 evenly sized slices, about ¼-inch each. (Here's How to Cut an Avocado.)
Assemble. Arrange the Ahi, mango, and avocado slices evenly between two appetizer plates -- I like alternating them, but arrange them any way you'd like to. Drizzle with a bit of Tamari or soy sauce, and sprinkle with the chives.
Serve. Serve immediately, or cover the plates with plastic wrap, drizzle the avocados with a bit of lemon juice, and refrigerate them until you're ready. (Up to an hour or so.)