How to Make It
Combine first 5 ingredients in a medium bowl. Divide beef mixture into 4 equal portions, lightly shaping each into a 1/2-inch-thick patty. Cover and refrigerate 20 minutes.
Heat a large, heavy skillet over high heat. Add oil; swirl. Add onion; cook 2 minutes, stirring constantly. Reduce heat to medium; cook 12 minutes or until caramelized, stirring occasionally.
Combine mayonnaise, relish, ketchup, and mustard in a small bowl.
Heat a grill pan over medium-high heat. Coat with cooking spray. Add patties; cook 3 minutes. Turn patties over; cook 2 minutes. Top patties with cheese; cook 1 minute or until desired degree of doneness. Divide caramelized onion among bottom halves of buns. Top with patties. Spread mayonnaise mixture evenly on top halves of buns. Top burgers with top halves of buns.
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