Soak kombu in 1 cup water at least 2 hours or overnight. Drain. Bring kombu and 6 cups water to boil in large saucepan. Remove kombu; discard. Add bonito; boil 2 minutes. Remove from heat; let sit 5 minutes. Mix in mirin, 1 1/2 teaspoons tamari, vinegar, sugar, and salt. Strain into medium saucepan. Add juice concentrate, ginger, and 3 teaspoons tamari; bring to simmer. Set broth aside. Heat 4 teaspoons oil in large nonstick skillet over medium-high heat. Add onions; saute 2 minutes. Transfer to bowl. Heat 2 teaspoons oil in same skillet over high heat. Add tuna; sear 2 minutes per side. Meanwhile, bring broth to simmer. Mound 1/2 cup rice in each of 4 shallow soup bowls. Ladle 3/4 cup broth around rice in each bowl. Top rice with ahi and onions. *Available at Japanese markets and specialty foods stores, and in the Asian foods section of some supermarkets.