High-quality fresh tuna or fresh salmon can be used in place of the toro, which is tuna from the fattier belly area.
Chop the fish very finely by hand until it holds together. In a medium mixing bowl, combine the chopped fish, minced garlic, and onion. Form the mixture into six 2-inch diameter cylinders with your hand or using a 2-inch cookie-cutter round as a mold. Place each of the completed cylinders into a small serving dish.
Combine the wasabi and soy sauce. Drizzle 2 tablespoons soy sauce-wasabi mixture around each cylinder. Garnish each dish as desired with green onion, chives, and a small spoonful of caviar.