Place potatoes in a heavy saucepan with enough water to cover. Bring to a boil, and cook until tender when pierced, 20 to 25 minutes. When cool enough to handle, cut into 1/4-inch thick slices. Place in a bowl. Meanwhile, make the dressing: In a small skillet over medium heat, fry the bacon until just crisp, 5 to 6 minutes. Add 1/2 C. of the onion and sauté until just tender but not brown, 2 to 3 minutes. Remove from the heat. In a small bowl, stir together the flour, sugar, salt and pepper. Stir in 1/4 C. of the vinegar and 1/2 C. water until smooth. Taste and add more vinegar if the mixture is not tart enough. Add the flour mixture to the bacon, then simmer over low heat, stirring, until slightly thickened, 4 to 5 minutes. Pour the hot dressing over the potatoes and add the remaining 1/4 C. onion, the parsley and the celery seed. Toss the salad by pouring it from one bowl to another until the potatoes are well coated. Serve the salad immediately, or let cool, cover, and refrigerate overnight, then serve cold.