Ready in 45 minutes
In a small saucepan over medium high heat, bring the 1/4 c apple cider to a boil. Cook, 4-5 mins or until it's reduced by half. Remove from heat and let cool. Once cider has cooled, transfer to a small bowl; add mustard and tarragon. Stir to combine and set aside.
Set up grill for direct cooking over high heat. Oil grate wihen ready to start cooking.
For burgers, in a large bowl, stir to combine ground sirloin, the remaining 1 T cider and the the onion soup mix. Form into 4 mini-patties; set aside. Spread cracked pepper on a plate. Carefully press both sides of the burgers into cracked pepper. Place burgers on hot oiled grill and cook 3-4 mins per side for medium. To serve, place hot burgers on toasted Kaiser rolls. Top with lettuce, onion, tomato, and mustard sauce.