Step 1: Place the cubed potatoes in a large pot. Cover with cold water by about an inch. Add a generous pinch of salt (about 1 tablespoon) to the water. Bring to a boil, then reduce heat and simmer until the potatoes are fork-tender, about 10-15 minutes. Be careful not to overcook them.
Step 2: Drain the potatoes thoroughly in a colander. Let them sit for about 10-15 minutes to cool slightly and allow excess steam to escape. This helps prevent a watery potato salad.
Step 3: While the potatoes are cooling, prepare the dressing. In a large mixing bowl, whisk together the mayonnaise, Dijon mustard, yellow mustard, apple cider vinegar, and granulated sugar (if using).
Step 4: Stir in the salt, black pepper, onion powder, and celery salt into the dressing. Taste the dressing and adjust seasonings as needed to achieve your preferred balance of tangy, sweet, and savory.
Step 5: Add the slightly warm, drained potatoes to the bowl with the dressing. Add the finely diced celery and red onion.
Step 6: Gently fold all the ingredients together with a rubber spatula until the potatoes are evenly coated with the dressing. Be gentle to avoid mashing the potatoes.
Step 7: Cover the bowl tightly and refrigerate for at least 2 hours, or ideally overnight, to allow the flavors to meld and deepen. This chilling time is crucial for the best classic tangy mustard potato salad.
Step 8: Before serving, give the potato salad a final stir and taste test. Adjust salt and pepper if necessary. Garnish with fresh chopped parsley or chives.