CUT 4-inch length from green end of 1 onion; slice remaining onions. Beat cream cheese and 1-1/4 cups cheddar with mixer until blended. Stir in sliced onions and peppers. Refrigerate 1 hour.
SHAPE into ball; roll in remaining cheddar. Cut pepperoni into shapes for the jack-o'-lantern's eyes, nose and mouth; press into cheese ball to make face. Insert green onion piece into top for stem.
SERVE with crackers.