Boil potatoes in water seasoned with coarse salt until they are tender, about 10 minutes. Drain cooked potatoes and return to the warm pot to dry them out. Let the potatoes stand 2 minutes, then spread potatoes out on to a cookie sheet to quick-cool them.
In the bottom of a medium mixing bowl, grate 1/4 onion using a hand grater. Add mustard and mayonnaise, paprika and cayenne pepper sauce to the grated onion and stir to combine. Add potatoes to the bowl and stir to evenly distribute dressing. Season potato salad with salt and pepper and taste to adjust your seasonings. Top servings of salad with chopped scallions.