Add potatoes to pot of salted cold water. Bring to a boil, over high heat, then reduce heat to a simmer and cook for 20 minutes until tender. Drain and place in a large bowl.
Add bacon to large cast iron skillet and cook until crisp over medium heat. Remove to a plate and reserve. Add potatoes and whites from the green onions to the bacon grease and sauté until potatoes have a little color and crisp edges.
Whisk together the vinegar, mustard, and sugar, salt, and pepper in a medium bowl.
Pour mixture over the potatoes and gently fold in with half the green onion tops and half the bacon. Taste for seasoning and adjust as necessary. Top with creme fraiche for serving and garnish with remaining green onions and crisp bacon.