Place scrubbed potatoes in a large pot and cover with cold water by about 2 inches. Add 1 tablespoon kosher salt and bring to a boil over high heat.

Reduce heat to maintain a gentle boil and cook potatoes until tender when pierced with a fork, about 20-25 minutes depending on size. Drain well and let cool just until you can handle them.

While potatoes cool, make the dressing: In a medium bowl, whisk together mayonnaise, sour cream, Dijon mustard, and apple cider vinegar until smooth and well combined.

Place cooled potatoes on a large baking sheet or cutting board. Using the bottom of a glass or measuring cup, gently press down on each potato until it cracks open and flattens slightly, creating a smashed texture with some chunks remaining.

Transfer smashed potatoes to a large mixing bowl. Add diced red onion, chopped dill, chopped parsley, and black pepper. Pour dressing over everything.

Using a large spatula or spoon, gently fold everything together until potatoes are evenly coated with dressing and herbs. Be careful not to overmix—you want to maintain the texture.

Taste and add additional salt if needed. Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.