How to Make It
Combine sugar, vinegar, and red pepper in a small bowl; stir well. Add cucumber and green onions; stir well. Cover and chill 2 hours.
Spread honey mustard evenly over 6 rye bread slices, and top each with sliced roast beef, about 1/4 cup cucumber mixture, and 2 tomato slices. Top each with remaining rye bread slices.