In a large bowl, whisk together 2 tablespoons each low-fat buttermilk and reduced-fat sour cream, and 1 tablespoon each cider vinegar and chopped chives. Season with coarse salt and ground pepper.
Toss 3 bunches (13 ounces total) arugula, well rinsed and stems trimmed, with dressing. Season with salt and pepper, if desired.
Divide evenly among 6 plates; top with Buttermilk Onion Rings, if desired.