Combine the vinegar, soy sauce, Worcestershire sauce, and mustard seeds in a large resealable plastic bag. Add the steaks and seal the bag. Refrigerate at least 30 minutes and up to 2 hours, turning the steaks occasionally.
Prepare the barbecue for medium-high heat or preheat a grill pan over medium-high heat. Drain the marinade from the steaks. Grill the steaks to desired doneness, about 6 minutes per side for medium-rare. Let the steaks rest for 10 minutes. Cut the steaks crosswise into thin slices.
Meanwhile, stir the cheese and sour cream in a small bowl to form a thick spread. Season the cheese mixture, to taste, with salt and pepper. Remove some of the bread from the center of the baguette halves. Spread the cheese mixture over the baguette halves. Arrange the rocket then the warm steak slices over the baguette bottom. Sprinkle the steak slices with salt and pepper. Cover with the baguette top. Cut the sandwich crosswise into 6 equal pieces and serve.