Makes 16 balls.
Melt chicken fat in small skillet over medium-low heat. Add onions and sauté 30 seconds. Set aside. Whisk eggs and all remaining ingredients in medium bowl to blend well. Stir in onion mixture. Cover; chill at least 6 hours and up to 1 day.
Bring large pot of salted water to boil. Drop matzo ball mixture by rounded tablespoonfuls onto foil, forming 16 mounds. Using wet hands, shape into balls; drop into pot. Cover; boil matzo balls until very tender, about 1 hour 10 minutes. Transfer matzo balls to shallow dish. (Can be made 1 day ahead. Cover; chill. Steam 15 minutes to rewarm.)