In a small bowl combine mayonnaise dressing, Dijon-style mustard, and horseradish. Slice bread shells or flatbreads in half horizontally. For each sandwich, spread one side of bread shell or flatbread with mayonnaise mixture. Top each with roast beef, red peppers, cheese, watercress, spinach, and remaining half of bread.
Wrap sandwiches in plastic wrap and refrigerate up to 24 hours. (Or, serve immediately.) To serve, slice sandwiches in half. Makes 12 servings (half sandwich per serving).