Turkey Burgers with Caramelized Onions, Cheddar, and Peach Chipotle Ketchup

My friend Bennett told me about a burger he used to eat in college so often that the diner renamed it the Benny Burger. It was a beef patty with grilled onions, melted cheddar and ketchup. These days, he’s not a red meat eater, so I felt inspired to give the original a makeover. I didn’t used to be much on turkey burgers, but this grown-up Benny Burger changed my mind.