In a medium size mixing bowl, whisk together buttermilk and mayonnaise. Add remaining ingredients and seasonings. Continue to whisk the mixture until the ingredients are fully incorporated and no lumps remain.
Pour into an airtight container and chill for at least 4 hours before serving. Store chilled for up to 5 days.
To make a thicker dressing, decrease the buttermilk by ¼ cup and increase the mayonnaise by ¼ cup.