Fontina and Caramelized Onion Stuffed Burgers

1. Heat the olive oil in a pan over medium heat. Add the sliced onion and cook until caramelized. 2. Divide the ground beef into four equal parts. Flatten two parts into a patty shape. 3. Place the caramelized onions and fontina cheese on the two patties. 4. Flatten the remaining beef and place on top of the filled patties. Seal the edges. 5. Cook the burgers on a grill or pan until desired doneness. 6. Serve on whole grain buns with your choice of toppings.