Season the steak with salt, pepper and the Old Bay. Preheat a grill pan or griddle to medium-high heat, greasing it. Grill the meat, turning occasionally, for 12 minutes. Transfer the meat to a cutting board and let rest for 10 minutes, then thinly slice against the grain. Wipe off the grill pan and turn the heat to low.
Meanwhile, rinse the sauerkraut and squeeze out the excess liquid. In a medium saucepan, heat the sauerkraut over medium heat.
In a small bowl, stir the sour cream, ketchup, relish and horseradish; season with salt and pepper.
Butter 1 side of each bread slice. Spread mustard on the other side. Build each sandwich with a bread slice, buttered side facing out, 2 cheese slices, some watercress, sauerkraut, steak, the homemade russian dressing and another bread slice, buttered side out. Grill the sandwiches to melt the cheese. Serve with the chips. Pass extra sauerkraut and dressing at the table.