In a large kettle, cook potatoes in boiling water until tender. (Potatoes can be peeled and cubed before boiling to save cooking time.) Drain and cool.
Separate hard-cooked egg yolks from the whites; set yolks aside. Chop whites and add to potatoes in a bowl. Add onion.
In a bowl, mash yolks. Stir in mayonnaise, milk, vinegar, mustard, sugar, salt and pepper. Pour over potatoes and toss well. Garnish with egg slices and paprika. Refrigerate.
