Bring a large saucepan of water to a boil over medium-high heat. Fill a large bowl with ice and cold water. Cook tuna in boiling water for 20 seconds, remove from water, and cool in ice water. Cut into sashimi-sized pieces, small squares about 1-inch by 2-inches. In a small bowl, mix together vinegar, miso, sugar, mirin, sake, and mustard. Pour sauce on top on tuna. Grill scallions until tender and lightly browned. Serve with grilled scallions on the side