Slice ½ inch off both ends of the onion. Pierce onion skin in several places with a sharp knife. Place onion in a microwave-proof bowl.
Add a tablespoon of water and microwave, covered, on high power until soft, about 3-4 minutes. Set aside to cool.
When cooled, peel and place in a food processor. Puree until smooth. Add oil, vinegar, sugar, mustard, garlic powder, celery salt, salt and pepper. Process until smooth.
