Place the potatoes in a large pot and cover with water so that the potatoes are covered by 2 inches. Bring the water to a boil and add 1 tbsp of kosher salt. Continue to cook, boiling, until the potatoes are just fork tender, about 8 minutes.
Meanwhile, fry the bacon in a skillet over medium heat until crisp, about 4 minutes per side. Transfer the cooked bacon to a paper towel lined plate and set aside. Discard all but 2 tbsp of the bacon fat (which keep in the same skillet you cooked the bacon in).
Add the diced onion to the bacon fat and cook over medium heat, stirring, until just tender.
In a bowl, whisk together the dijon mustard, whole grain dijon mustard, apple cider vinegar, distilled white vinegar, parsley, scallion, garlic, pepper and cayenne pepper (if using). Pour into the hot skillet with the sauteed onions and stir to combine. Add the sauerkraut and crumble the bacon into the mixture and cook, gently stirring, until just warm, about 4 minutes.
When the potatoes are done, drain and let slightly cool. Place in a large serving bowl and pour the mustard sauce over the potatoes. Gently toss to combine. Add plenty of salt (I ended up using about 2 teaspoons here) and toss once more. Serve immediately.