Since it will not be cooked, purchase sushi-grade tuna from your local fish market. It should be firm, shiny, plump, and bright red with no fishy odor. Keep the tuna mixture well chilled, and assemble the appetizers just before serving. The tuna can also be seared, if you prefer: Cook the whole fillet in a pan over high heat for 1 minute on each side; cool and cut into 1/4-inch cubes. If you're a fan of wasabi, substitute wasabi paste for the horseradish.