Boil the potatoes in lightly salted water. Cut the pepper into thin 2-inch strips. Cut the onion into thin slices. Slice the warm boiled potatoes into a salad bowl, scatter the sliced peppers and onions over them and pour on about 1/2 cup of the vinaigrette.
Using your hands, gently mix the salad until each potato slice is coated with vinaigrette and the onions and peppers are well distributed.
Taste the salad and season it with salt and freshly ground pepper.
Let the salad stand at least an hour. Before serving, taste the salad again and add vinaigrette to taste.
Combine all ingredients in a bowl EXCEPT oil and whisk until frothy.
Whip mixture continuously as a steady stream of oil is added to the mixture. Whisk until all oil is incorporated.