This grilled cheese burger was inspired by the Sonic Grilled Cheese Double Burger, a delicious mash up of a grilled cheese and a cheeseburger. The patties are thin, slightly crispy, and irresistible.
Melt butter to a large cast-iron pan over low heat. Add the bread to the pan and cook for 1-2 minutes on each side, or until lightly toasted. Remove the toast from the pan.
Melt more butter in the pan. Add diced onions and cook for 5-6 minutes, until translucent and slightly golden-brown. Once cooked, remove them from the pan and set aside.
Roll the ground beef into 6 balls. Place 1 or 2 balls into the hot cast-iron pan, cover with a square of parchment paper, and press down with a large metal spatula or burger press until thin.
Cook for 1-2 minutes, until slightly crisp on the bottom, then flip, add cheese, and cook for another minute. Remove from the pan and repeat with the remaining patties.