In small bowl whisk together mayonnaise sour cream water vinegar and salt
Set aside
Cut red onions crosswise in half
To make onion cups cut and scoop out each onion half leaving bottom intact and about 14-inch thick shell Finely chop enough scooped-out onion to make 14 cup
Add chopped onion to mayonnaise mixture
Fill red onion cups with dressing
Garnish with chives if desired and refrigerate while preparing salad