1. Slice bread in half lengthwise. Scoop out soft bread center; discard.
2. Combine mayonnaise, mustard, onion and horseradish in medium bowl. Spread mixture on halves of bread. Layer remaining ingredients on bottom half of bread. Cover with top half of bread, pressing gently. Wrap in plastic wrap; chill in refrigerator 30 minutes or until ready to serve. Cut into thick slices to serve.