Dice tuna into 1/3-inch cubes, discarding fibrous tissue, and place in a glass bowl. Toss tuna gently with olive oil and next 3 ingredients. Cover and refrigerate up to 2 hours.
Just before serving, add green onion and next 3 ingredients, and toss. Season to taste with salt and pepper. Arrange in martini glasses, and garnish, if desired. Serve immediately.
Chef John Recommends: An aromatic Sauvignon Blanc. Alternative Pours: A Gewürztraminer, Riesling, Viognier, or Muscat to complement the mango.