Simmer whole potatoes in salted water until tender, about 20-30 minutes, and cook eggs using preferred method for hard boiled. Drain and set aside to cool.
Whisk together the mayonnaise, mustard and sugar until sugar is dissolved. Set aside.
Peel potatoes and eggs. Slice thinly using box grater over large bowl, alternating potatoes and eggs (slice a potato then slice an egg until everything is sliced). Season with salt and pepper and add diced onions to bowl.
Add dressing mixture to potatoes and eggs and mix gently until well combined. Refrigerate several hours or overnight for best flavor!
