In a medium bowl, whisk together mayonnaise and buttermilk until smooth. Add onion, garlic, mustard, dill, parsley, and celery seeds, if using, and whisk until well combined. Let rest 5 to 10 minutes for flavors to meld. Serve immediately or refrigerate up to 3 days until ready to use.
Source: Hannaford fresh Magazine, September - October 2010