Place a large nonstick skillet over medium-low heat. Add butter and vegetable oil. When hot, add the onion.Cook stirring occasionally for 25 to 30 minutes.Season with salt and pepper.
Cut the potatoes in quarters and place in a large pot. Add water to cover by 1-inch. Bring to a boil and then add 1 tablespoon of salt.Reduce heat to a simmer and simmer for 15 to 20 minutes or until tender. Drain.
In a large bowl, stir together mayonnaise, sour cream, mustard, vinegar, salt, and pepper.
Add onions and potatoes and mix well. Sprinkle bacon and parsley on top.
Refrigerate for at least 2 hours before serving.
Notes
Boil the potatoes while the onions cook to save time.