Cook the potatoes.Add the potatoes to a large stockpot, cover with cold, well-salted water, and bring to a boil. Cook until fork-tender.
Make the dressing.While the potatoes cook, combine the mayonnaise and whole grain mustard in a large mixing bowl.
Build the flavor.Stir in the red onion, dill relish, capers, dried dill, and lemon juice until well combined.
Drain and dry.Once the potatoes are tender, drain them well and let them sit in the colander for a few minutes to steam off excess moisture. This helps the salad stay creamy, not watery.
Combine.Add the warm potatoes directly to the bowl with the dressing. Use a fork to gently break them into bite-sized pieces while folding everything together. Stop when the potatoes are slightly smashed and evenly coated.
Season and serve.Taste and adjust with kosher salt and pepper as needed. Serve warm or chill in the fridge before serving.