For burgers, in a large bowl, stir to combine ground sirloin, the remaining 1 tablespoon cider, and the onion soup mix. Form into 4 mini-patties; set aside. Spread cracked pepper on a plate. Carefully press both sides of burgers into cracked pepper.
Place burgers on hot, oiled grill and cook 3 to 4 minutes per side for medium (160 degrees F). To serve, place hot burgers on toasted Kaiser rolls. Top with lettuce, tomato, onion, and mustard sauce.