Cook 1 1/2 pounds potatoes in large pot of boiling salted water for around 30 minutes, or until a fork can skewer through the potato with ease but still a little resistance. Drain and cool, then cut potatoes into chunks and transfer to large bowl.
Add celery, onion, chives, parsley, and egg to the potatoes, tossing the ingredients until well combined.
In a separate bowl or a liquid measuring cup, combine 1/3 cup mayonnaise, 1 generous squirt Dijon mustard, 1 tablespoon apple cider vinegar, a pinch of granulated sugar, and a pinch of salt. Stir with a fork or a whisk until well combined, tasting as you go and adding more of whatever ingredient you want to dominate the potato salad. If you've added too much sugar, no big deal! Add a little more apple cider vinegar and salt. If you've added too much salt, no big deal either — go the opposite way and add more sugar. Too mayonnaisey? Add more mustard. Too mustardy? Add more mayo. Play around until you figure out what you like!
Pour the dressing over the potatoes, and toss until well coated and dressed. Season to taste with salt and pepper.