Preparation Combine cooked steak, capers, cornichons, red onion, Dijon mustard, and 2 tablespoons mayonnaise in medium bowl; toss to blend. Season salad with salt and pepper. (Can be prepared 6 hours ahead. Cover and refrigerate.) Arrange bread on work surface. Spread slices with remaining 3 tablespoons mayonnaise. Divide steak salad among 4 bread slices. Top salad on each with 2 tomato slices and 1/4 of arugula. Press second bread slice, mayonnaise side down, onto each sandwich. Cut sandwiches diagonally in half and serve.
Preparation Combine cooked steak, capers, cornichons, red onion, Dijon mustard, and 2 tablespoons mayonnaise in medium bowl; toss to blend. Season salad with salt and pepper. (Can be prepared 6 hours ahead. Cover and refrigerate.) Arrange bread on work surface. Spread slices with remaining 3 tablespoons mayonnaise. Divide steak salad among 4 bread slices. Top salad on each with 2 tomato slices and 1/4 of arugula. Press second bread slice, mayonnaise side down, onto each sandwich. Cut sandwiches diagonally in half and serve.