In a small bowl, combine mayonnaise, sugar-free ketchup, Dijon mustard, apple cider vinegar, and a pinch of salt and pepper. Whisk until smooth; set aside.

Heat a large skillet over medium heat and melt 2 tablespoons of butter. Optionally, add a splash of olive oil. Once sizzling, add the thinly sliced corned beef, stirring frequently for 3-5 minutes.

Add about 1 cup of rinsed sauerkraut to the skillet, stir to combine, and cook for an additional 3-4 minutes. Adjust seasoning with salt and pepper if needed.

Reduce heat to low, sprinkle 1 cup of shredded Swiss cheese over the mixture, cover with a lid, and cook for 2-3 minutes until melted.

Remove from heat, divide into bowls, drizzle with dressing, and garnish with chopped parsley. Serve immediately.