In a small sauce pan, melt the butter over medium heat. Cook the onion and garlic until the onion is very soft and just beginning to brown. Remove from heat.
In a mixing bowl, combine the cooked onion mixture, chives, cream cheese, sour cream, mayo, mustard, and celery seeds. Mix well. Season with salt and pepper to taste.
This dip is great to make the morning or even day before your event because the flavors will deepen. Simply store in an airtight container in the refrigerator until you're ready to serve.