Cook the onions in the bacon grease over medium heat until softened and starting to brown, about 5 to 8 minutes. Add vinegar, water, sugar, salt, pepper, celery seeds, and mustard. Bring to a boil, then add potatoes, parsley, and half of the crumbled bacon. Toss to coat the potatoes and evenly heat everything through, then transfer to a serving dish. Sprinkle with the remaining crumbled bacon and chopped parsley. Serve warm.
1 medium onion, 1/2 cup white vinegar or apple cider vinegar, 1/4 cup water, 4 Tablespoons granulated sugar, 1 teaspoon table salt, 1/4 teaspoon freshly ground black pepper, 1 1/2 teaspoons celery seeds, 1 Tablespoon dijon or spicy brown mustard, 2-3 Tablespoons chopped fresh parsley