Boil or steam the potatoes until just done; do not overcook. Drain.
Allow potatoes to cool slightly, then cut into one-fourth-inch slices into a large serving bowl. Heat the butter in a nonstick pan and add the onions. Cook just until soft and transparent. Stir in the vinegar, water, black pepper, parsley and celery seeds. Stir and cook for about 2 minutes more. Pour the sauce over the hot potatoes. Stir gently and serve.
Makes 6 servings