Preheat oven to 425?F. Pat tuna steaks dry with paper towel. Stir together miso, sugar, sesame oil, soy sauce, vinegar and sriracha.
Heat oil in large skillet set over high heat; cook tuna, turning once, for about 5 minutes or until golden brown on both sides.
Brush miso glaze evenly over steaks.
Place on parchment paper–lined baking sheet.
Bake in center of oven for 3 to 5 minutes or until medium-rare in center; let stand for 10 minutes.
Flake into large pieces with fork.