Wash and rinse the potatoes well. Leave the peels on for the cooking process.
Dice the yellow onion, mince the garlic, and chop the parsley, cut the bacon into lardons, slice the green onions, and place in prep bowls.
Measure out the beef stock, apple cider vinegar, sugar, and mustards into prep bowls.
Boil the Potatoes
Add the potatoes to a large pot and cover by 2 inches or so with cold water. Add enough Kosher salt to make the water taste slightly salty. Cook on high until you achieve a roaring boil. Reduce the heat slightly and let boil until a fork pierces the potatoes easily. Remove the potatoes from the heat and drain. Let cool until they are cool enough to touch and peel.
Make the Sauce
While the potatoes are cooking, add the bacon to a cold skillet and set to medium-high. Cook the bacon until crispy, and then remove with a slotted spoon or spider strainer onto paper towels. Set the bacon aside.
Leaving the bacon fat in the skillet, add the yellow onion and cook until softened. Add the garlic and cook for a minute or so. Add the beef stock, apple cider vinegar, sugar, and mustards. Bring to a boil, add the bacon back to the skillet, and then reduce to a simmer. Let simmer until you finish peeling and cutting the potatoes.
Peel and Slice the Potatoes
Once the potatoes are cool enough to handle, peel them and then chop into bite-sized pieces. Or, you can slice the potatoes into 1/4 inch slices and add the potatoes to a large mixing bowl.
The Sauce Meets the Potatoes
Remove the sauce from the heat and add slowly to the potatoes. Mix gently to combine. Add the parsley and mix again to combine. Check for seasoning and add Kosher salt and pepper to taste.
Garnish and Enjoy!
Sprinkle the green onions over the potato salad and serve warm. This salad is delicious cold, too! Enjoy!