Prepare the meat: Combine the mustard seeds, juniper berries, peppercorns, allspice berries and bay leaves in a small skillet over medium heat; cook, swirling the skillet, until toasted, about 3 minutes. Transfer to a bowl to cool, then grind in a spice grinder with the cinnamon, ginger and 1 teaspoon salt. Coat the steak with the spice mixture and refrigerate at least 3 hours or overnight.
Make the sauce: Combine the mayonnaise, ketchup, horseradish, vinegar, Worcestershire sauce and the shallot in a small bowl.
Cook the meat: Heat a large cast-iron skillet over medium-high heat and add the vegetable oil. Add the steak and cook 3 to 5 minutes per side for medium rare. Transfer to a cutting board and let rest 5 minutes.
Meanwhile, reduce the heat under the skillet to low. Add the sauerkraut and sprinkle with the caraway seeds; cook, tossing, until warmed through. Spread the sauerkraut into a thin layer and top with the cheese; let melt.
Spread the rolls generously with the sauce. Thinly slice the steak against the grain and layer on the roll bottoms. Top each with some of the cheese-covered sauerkraut, sprinkle with the fried onions and cover with the roll tops.