To serve as a meal for 4 later in the week, reserve ½ of roasted beef. Stir together ¼ cup light mayonnaise, 1 tbsp prepared horseradish, and 1 tsp lemon juice. Season with salt and pepper. Spread on 8 slices whole-grain bread. Very thinly slice reserved beef and add to sandwiches, along with tomato slices and lettuce.