Melt the butter in a large, deep skillet over medium-high heat. Add the thinly sliced onions and cook, stirring frequently, for about 10 minutes until they begin to soften.

Lower the heat to low, sprinkle in the kosher salt, and continue to cook the onions, stirring every 5 minutes, until they have reduced and turned a rich golden-brown—about 35 to 40 minutes. Remove from the heat and set aside.

While the onions caramelize, cook the hot dogs using your preferred method—grill, pan-fry, or boil—following the package directions.

Once the hot dogs are cooked and the onions are caramelized, place each hot dog in a bun.

Top each hot dog with a generous spoonful of the caramelized onions, followed by a layer of sauerkraut, and finish with a drizzle or dollop of stoneground mustard.

Serve the assembled sauerkraut hot dogs immediately. Any extra mustard, sauerkraut, or onions can be stored separately and used later.