Cook 4 slices bacon in a large skillet over medium heat, turning occasionally, until crispy, about 8 minutes. Transfer to a paper towel–lined plate to cool, reserving the drippings. Crumble the cooled bacon and refrigerate until ready to serve.
Discard all but 2 tablespoons of the drippings. Off the heat, add ⅓ cup vinegar to the pan. When it stops steaming, scrape up any browned bits. In a small bowl, whisk 3 tablespoons mustard and 2 tablespoons flour; add the vinegar mixture and whisk until combined. Whisk in ½ teaspoon salt and ½ teaspoon pepper.
Combine the halved (or quartered) potatoes, 2 cups celery, the chopped onion and the dressing in a 5- to 6-quart slow cooker; stir until the vegetables are well coated. Cover and cook on Low for 5 hours until the potatoes are tender.
To serve, stir in ¼ cup dill and the reserved bacon; serve warm.
