The rye bread in this sandwich packs a good amount of Resistant Starch, and the veggies contribute plenty of fiber to keep you feeling satisfied all afternoon long.
Combine mayonnaise and horseradish in a small bowl; stir well with a whisk.
Spread mayonnaise mixture on bread slices. Top 1 bread slice with roast beef, spinach, and remaining bread slice.
Serve with cucumber slices drizzled with dressing.