Transfer onions to the base of the Power Chef® System fitted with blade attachment. Let stand 10 minutes. Cover and pull cord until onions are well minced.
Replace blade attachment with paddle attachment. Add sour cream, salt, cream, vinegar, garlic powder and mayonnaise. Cover and pull cord until well blended.
Remove paddle from Power Chef® System. Cover and refrigerate at least 10 minutes, up to overnight. Serve cold as a dip with vegetables, chips, or pita wedges, or serve as a sauce for steak, chicken, or fish.