This summer staple has no mayonnaise, so it's perfect for an outdoor lunch.
Place unpeeled potatoes in a large pot. Cover with cold water and bring to a boil. Turn down heat and simmer gently until potatoes are tender when pierced with the point of a knife, about 35 minutes. Drain and refresh in cold water.
In a nonstick skillet, cook bacon until very crisp. Drain on paper towels. When cool, crumble into large pieces. Set aside.
In a small bowl, whisk together mustard, vinegar, sugar, and salt and pepper to taste. Whisk in oil and set aside.
Cut potatoes into 1/4-inch-thick slices and place in a large bowl. Stir in onion and dressing. Let marinate at least 1 hour.
Before serving, stir in parsley and bacon, reserving some to sprinkle on top.
The potato skins will stay on better if you don't salt the cooking water.