In a large saucepan combine citrus juices, tamari or soy sauce, vinegar, oil, and gingerroot and simmer 15 minutes. Pour sauce through a fine sieve into a bowl. (Sauce may be made 1 day ahead and chilled, covered. Reheat sauce before preceeding.)
Preheat oven to 400°F..
In a large non-stick skillet, heat oil over moderately high heat until hot but not smoking. Coat 2 tuna steaks with sesame seeds and sear 1 minute on each side, transferring to a baking sheet. Coat and sear remaining 4 tuna steaks in batches in same manner. Cook tuna in middle of oven 3 minutes for medium-rare.
Cut each tuna steak into 4 slices and arrange slices, cut side up, on 6 plates. Pour about 1/4 cup sauce around tuna on each plate.
