Mix lemon or lime juice, soy sauce, olive oil, tahini, ginger and garlic in a mini food processor. (Reserve a little marinade to lightly dress a vegetable side dish.) Place marinade together with fish in ziplock bag and marinate in refrigerator for 1 hour, turning occasionally. Remove from marinade. Preheat broiler. Place swordfish 4 inches from heat source on a lightly oiled, flat broiling dish. Dont place fish directly on broiling rack, which can break it apart. Brushing with reserved marinade, broil fish to desired degree of doneness, 4-5 minutes per side. Or, grill swordfish. Dont overcook. Sprinkle with scallion and garnish with lemon. Serve immediately.