Wash the potatoes and drain. Using a paring knife, trim a band of the peel about 1/2 inch wide (depending on the size of the potatoes) around the circumference of each potato. This will form an attractive "belt". Boil the trimmed potatoes gently in ample, lightly salted water until just tender. The potatoes are done as soon as they pierce easily with a sharp knife. Carefully drain, spread on a tray and allow to cool.
Cook the bacon in a medium sized frying pan until almost crisp.
Add the onion and cook until limp.
Drain off any excess fat.
Place the cooled potatoes in a bowl along with the cooked bacon, onion and the scallions.
In another bowl, blend together the mayonnaise, dry mustard, vinegar, oil, salt and pepper to taste then add to the bowl of potatoes and toss together. Fold in the hard-boiled eggs. Cover and refrigerate.