Prepare teriyaki sauce.
In a bowl, combine beef, bread crumbs, onions, egg, and 3 tablespoons of the teriyaki sauce.
Shape into balls about 1 inch in diameter.
Arrange them in a single layer on a rimmed baking sheet. (At this point you may cover and refrigerate for up to 8 hours.)
About 10 minutes before serving, pour remaining teriyaki sauce over meatballs and roll to coat evenly.
Lift about 12 meatballs at a time from marinade and arrange in a circle on a flat 10 inch plate.
Microwave, uncovered, on HIGH (100%) for 3 to 3 1/2 minutes (rotating plate 1/4 turn after 1 1/2 minutes) or until meat is done to your liking when slashed.
Repeat with remaining meatballs.
Makes about 4 dozen appetizers.
Teriyaki sauce.
In a small bowl, combine 1/3 cup soy sauce, 2 tablespoons each sugar and dry sherry, 3/4 teaspoons grated fresh ginger (or 1/4 teaspoon ground ginger), 1/4 cup finely minced green onions, 2 cloves garlic (minced or pressed), and 1/4 teaspoon pepper.