Preparation

• Mix  sour cream, mayonnaise, chives, horseradish and mustard until blended. Refrigerate until ready to use.
• Cut  onions into 1/2-inch-thick slices; separate into rings. Set aside 20 onion rings that are 2-1/2 to 3 inches in diameter. (Reserve and refrigerate remaining onions for another use.)
• Heat  2 inches oil in deep-fat fryer to 375°F. Meanwhile, whisk egg, milk and baking mix until blended; let stand 5 min.
• Combine  cracker crumbs and cheese in pie plate. Dip onion rings, 1 at a time, into batter, then into crumb mixture, turning to evenly coat both sides of each onion ring with each ingredient.
• Add  onion rings, in small batches, to hot oil; cook 2 to 3 min. or until crisp and deep golden brown. Use slotted spoon to remove onions from oil; drain on paper towel-covered baking sheets.
• Serve  onions rings with the sour cream mixture.