Preheat oven to 500 degrees F. Wrap a brick in aluminum foil and place into the oven to heat.
Place the game hen breast side down on a cutting board. Using scissors or poultry shears, cut from the neck to the tailbone to remove the backbone. Once you remove the backbone you will be able to see the inside of the bird. Make a small slit in the cartilage at the base of the breastbone to reveal the keel bone. Grab the bird with both hands on the ribs and open up like a book, facing down towards the cutting board. Remove the keel bone. Cut small slits in the skin of the bird behind the legs and tuck the drumsticks into them in order to hold them in place. Season on both sides with salt and pepper.
Fry the bacon in a 12-inch cast iron skillet over medium heat. Once crisp, remove the bacon from the skillet and reserve. Drain all but 1 tablespoon of fat from the pan. Immediately add the two birds to the pan, skin side down. Add the onions to the pan around the edges. Top the birds with the brick and allow to remain on the heat for 5 minutes. Place the pan into the oven and cook 10 to 15 minutes or until the thigh meat reaches 170 degrees F. Remove from the oven and allow the bird to rest for 5 minutes before serving with the onions and bacon.