Dice the onion.
Chop the scallions, dividing the white and light green parts in one pile and the dark green parts in another.
Chop the bacon.
Place potatoes in a large stock pot. Cover with cold water by about 2 inches. Add some salt to the pot and bring to a boil over medium high heat. Once boiling, reduce the heat to medium.
Boil the potatoes until fork tender. This will take about 15 to 20 minutes depending on the size of the potatoes. Strain the potatoes and set aside.
Cook the bacon in a skillet over medium heat until crispy. Remove the cooked bacon from the pan and set aside. Do not drain the bacon drippings.
Add the chopped onion to the bacon fat in the pan and cook until translucent. Add the white and light green portions of the scallions and cook for another minute.
Remove the pan from the heat, then add the vinegar and mustard and stir well.
Once the potatoes have cooled slightly, slice them in half (or in quarters for particularly large potatoes).
Add the potatoes to a bowl and pour the hot dressing over the potatoes. Add the dark green parts of the scallions, the fresh dill, and stir to combine.
Season with salt and pepper to taste and serve.