Add the cubed red potatoes to a large pot and cover with cold water by 1–2 inches (2.5-5 cm). Season the water generously with kosher salt. Bring to a boil over medium-high heat, then reduce slightly to maintain a even boil. Cook for 9–12 minutes, or until the potatoes are tender when pierced but still hold together. Drain well and transfer to a large mixing bowl. Let the potatoes cool for about 15–20 minutes so they are no longer steaming hot but still warm enough to take on the dressing.
2 lbs (900 g) red potatoes, cold water, 1½ tbsps kosher salt
While the potatoes cook and cool, cook the bacon in a skillet over medium heat until crisp. Transfer the bacon to a paper towel-lined plate and reserve about 2 tablespoons (30 ml) of the bacon grease in the pan. Add the minced yellow onion to the warm bacon grease and cook for about 4 minutes, stirring occasionally, until softened. Remove from heat.
½ lb (225 g) bacon, 2 tbsps (30 ml) bacon grease, 1 cup (150 g) yellow onion
In a small bowl, whisk together the apple cider vinegar, sugar, whole grain mustard, Dijon mustard, kosher salt, and coarse black pepper until smooth and fully combined. Using both mustards provides texture from the whole grain and a smoother base from the Dijon, allowing the dressing to come together more completely than a vinegar-only mixture.
⅓ cup (80 ml) apple cider vinegar, 3 tbsps (38 g) granulated sugar, 1 tbsp (15 g) whole grain mustard, 1 tbsp (15 g) Dijon mustard, 2 tsps (6 g) kosher salt, ¾ tsp (2 g) coarse ground black pepper
Add the cooked bacon, softened onions, and chopped parsley to the bowl of warm potatoes. Pour the dressing over the top and gently toss until the potatoes are evenly coated, taking care not to break them apart. Top with fresh chives if you like.
½ tbsp (2 g) finely chopped fresh parsley, fresh chives
Serve warm, or cover and refrigerate for at least 3 hours before serving cold.