Preheat oven to 425 F.
Place hasselbacked potatoes in a large baking dish (metal, ceramic, or pyrex preferred).
Drizzle oil over potatoes, open up the slits to allow oil to get inside. Sprinkle salt and pepper over potatoes.
Bake for 45 minutes until fork tender and edges begin to crisp. If you have larger potatoes you will have to extend your bake time.
Place beef bacon in a large (room temperature) pan on the stove top. Heat pan on medium high, as bacon fat renders down, it will begin to crisp up. Fry on both sides until crispy (about 5 minutes on each side). You may need to do this in batches and adjust heat as you go to avoid the grease splattering. Remove bacon from pan. Once cooled, chop or crumble bacon to create crispy bacon bits. Set aside.
Sauté diced onion in bacon fat until softened, 7-10 minutes.
Once onion is translucent, add water, sugar, and vinegar directly to the pan. Stir to incorporate everything, and simmer until reduced to a thick sauce, about 10-15 minutes. Stir often.
Add half of the bacon bits to the sauce.
The potatoes should be done at this point. Pour sauce over roasted potatoes, making sure to scoop sauce directly into the slits.
Return pan to the oven and bake for 15 minutes.
Remove pan, sprinkle remaining bacon and chopped parsley bits over potatoes and serve hot.