Cook onion in butter till tender.
Stir in crumbs, poultry seasoning, and salt and pepper.
Blend in 1/2 cup of the mayonnaise.
Cut fillets in 4 uniform strips, about 1 1/2x 8 inches.
Spread each with bread stuffing; roll up, jelly-roll fashion.
Place in greased custard cups.
Set cups in shallow pan.
Bake in moderate oven (350°) about 20 minutes.
Spread with rest of mayonnaise.
Bake 20 minutes more.