Celery-root and watercress give this hearty roast beef sandwich a nice crunch.
In a medium bowl, combine mayonnaise, mustard, lemon juice, capers, and horseradish; season with salt and pepper. Grate reserved celery root into dressing and stir to combine. Spread mixture on 4 slices country-style bread. Divide reserved watercress and roast beef among slices, then top with 4 more bread slices. (To store, refrigerate, wrapped in plastic, up to 1 day.)