Preparation Whisk mayonnaise, horseradish and rosemary in small bowl to blend. Season dressing with salt and pepper. Spread bottom piece of bread with half of dressing. Top with roast beef, tomatoes and arugula. Sprinkle with salt and pepper. Spread remaining dressing on top piece of bread. Press bread, dressing side down, onto arugula. Cut into 4 pieces and serve.
Preparation Whisk mayonnaise, horseradish and rosemary in small bowl to blend. Season dressing with salt and pepper. Spread bottom piece of bread with half of dressing. Top with roast beef, tomatoes and arugula. Sprinkle with salt and pepper. Spread remaining dressing on top piece of bread. Press bread, dressing side down, onto arugula. Cut into 4 pieces and serve.