In a small saucepan, whisk together the water and cornstarch, and cook over low heat until thickened and clear. Remove from the heat and set aside to cool slightly. In a blender or food processor, place the sugar, onion, dry mustard, and salt, and process for 10 seconds. Add the vinegar and lemon juice, and process for 1 minute or until smooth. Add the water-cornstarch mixture and celery seed, and process for a few seconds to incorporate. Transfer to an airtight container and store in the refrigerator for up to two week. If the dressing separates, stir well before serving.

Yield: 2 Cups