1. If you peel potatoes while they are still warm, they'll absorb flavors better. Cut them into 1-inch dice into a large bowl. Gently fold in the onion, vinegar, mustard, sugar, celery seed, and salt and pepper to taste. Let stand 30 minutes to an hour at room temperature.
2. Then fold in the celery, egg, and mayonnaise. At this point, it's best to let the salad rest a minimum of 1 hour or so in the refrigerator so flavors can meld. It can even be refrigerated up to 3 days. But because potatoes absorb and mute seasonings, taste it before serving and adjust vinegar and seasonings if needed.