Hollow out the center of each onion leaving a half-inch thick shell; set aside.
Finely chop center portions.
Combine onion, chicken and mayonnaise in medium bowl.
Season with salt and pepper.
Fill onion shells with stuffing and place in (8 x 8-inch) baking dish.
Dot each onion with 1 tablespoon butter.
Pour catsup around onions.
Cover with plastic wrap.
Microwave 9 minutes on high, or until onions are tender-crisp.