1. Heat grill. Place chicken breast on grill and cook for 4-5 minutes on each side. Place pineapple slice on grill and cook about 1 minute on each side.
2. Make horseradish aioli. In a small bowl, whisk together horseradish, garlic and mayo. Spread aioli on one side of each bread slice. Layer on Romaine, grilled chicken and pineapple. Top with second slice of bread, aioli side down.