Remove fillets from breast halves; reserve for another use. Starting with thinner edge, cut each breast in half lengthwise, horizontally, almost all the way through. Open up breasts and pound to 1/4-inch thickness. Combine 3 Tbsp. soy sauce, sherry and sugar in large shallow dish. Add breasts, skin side down; marinate 30 minutes.
Meanwhile, combine bread, celery, almonds and onions. Blend butter with remaining 2 Tbsp. soy sauce; pour evenly over bread mixture and toss together until moist and crumbly.
Remove turkey; reserve marinade. Spread 1/2 of stuffing mixture on 1/2 of each breast half, leaving a narrow border along outside edges; pack stuffing down slightly. Close breasts and secure with skewers or toothpicks; brush with reserved marinade. Place on rack in baking pan.
Roast at 325 degrees F 1 hour 15 minutes, or until tender; brush with reserved marinade every 30 minutes. Remove to serving platter; let stand 15 minutes. Just before serving, remove skewers and cut crosswise into 1/2-inch thick slices.