Place the Dijon mustard, sugar, vinegar, garlic clove, and salt into a blender. Cover, and puree until the garlic has blended smooth.
Sprinkle in the xanthan gum, and blend about 15 seconds to mix well. With the blender running, pour in the vegetable oil alternately with the water; blend until thick and creamy.
Add the celery seed, red pepper flakes, black pepper, tarragon, and mustard seed; pulse until the spices are evenly distributed.