Melt butter in large skillet on medium heat. Add onions; cook 20 min. or until golden brown, stirring frequently. Stir in vinegar; stir 5 min., stirring frequently. Cool.
Reserve ¼ tsp. parsley. Mix remaining parsley with mayonnaise, sour cream, and mustard in medium bowl until blended. Stir in onions. Refrigerate 30 min. or until chilled.
Garnish with reserved parsley. Serve with crackers.