¼ cup chopped half-sour pickles, plus 1 cup brine from the jar
3 pounds baby potatoes, halved (about 9 cups)
½ cup mayonnaise
¼ cup chopped fresh dill
3 tablespoons Dijon mustard
½ teaspoon freshly ground black pepper, plus more to taste
kosher salt to taste
Fill a large pot halfway with generously salted water; add ¾cup of pickle brine and bring to a boil. Add the potatoes and cook until easily pierced with a fork, 13 to 15 minutes. Drain and transfer to a sheet pan to cool to room temperature.
Whisk the mayonnaise, pickles, dill, mustard, remaining ¼cup of pickle brine, and the pepper in a large bowl until fully combined.
Add the cooled potatoes and gently toss to coat. Season with more pepper and kosher salt to taste. Serve immediately or store tightly covered in the refrigerator for up to 3 days.