Thoroughly pat steak dry with paper towels. Cut steak into thirds.
Place sugar, coffee, salt, mustard, and pepper in a small bowl; using your hands, mix until combined and sugar is broken down into fine crystals. Coat steak generously with sugar mixture, pressing to adhere. Wrap each steak piece tightly in plastic wrap; chill 1 hour.
Heat grill to high (450°F to 550°F). Unwrap steak pieces; pat dry with paper towels. Drizzle with olive oil. Grill, uncovered, until medium-rare, about 2 minutes per side. Transfer meat to a cutting board; cut across the grain into thin slices.
Note: For variety, I like to offer chicken sandwiches as well. Roast a chicken (or buy one already cooked), and slice the breast meat. Make the same sandwich, but omit the red onion and instead of adding horseradish and Creole mustard to the mayo, add chopped fresh tarragon or basil with a little minced scallion.