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!
To make more, for every potato used, use the same amount of eggs.
One side of the box grater is for slicing, it has the largest slits.
To hard boil the eggs in an Instant Pot, place 1 cup water in insert and wire basket. Add eggs and set for 5 minutes under high pressure. Naturally release for 5 minutes, open and remove eggs to ice water bath for 5 minutes. They are ready to go and peel so easily!
You can use Miracle Whip instead of mayonnaise, it will be sweeter so adjust the sugar amount if needed.