Boil or steam potatoes until just done; do not overcook. Drain. Allow the potatoes to cool slightly; then cut into one-fourth-inch slices and put into a large serving bowl. Melt butter in a nonstick skillet over medium heat. Add the onions, and cook, stirring, until soft and transparent. Stir in the vinegar, water, pepper, and celery seed. Stir and cook for about 2 minutes longer. Pour the mixture over the hot potatoes. Stir gently and serve.
Makes 6 servings