With a grapefruit knife, hollow out the center of each onion leaving a half-inch thick shell.
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.
For variety; substitute 1 can (10 3/4 ounces) double-strength chicken broth, undiluted, for catsup.
Can be prepared in advance and cooked later