Cook potatoes in boiling salted water until fork tender. I boiled my small potatoes about 10 minutes.
Drain and cut into chunks. Chop bacon and cook over medium low in a large skillet until crispy. Remove bacon and drain and remove all but 2 tbsp of the fat.
Add the onion and cook until lightly browned, about 5 minutes. Add in the vinegar, water, sugar, salt, pepper, and celery seed along with the mustard and allow to come to a boil. Add the potatoes and cook, stirring until the sauce has thickened a bit. Move to a bowl and toss with the bacon and parsley and serve warm or room temp.